I hope this will not come as a shock to US readers, but one of the constants of UK culture is to laugh at the ability of so many in the US to believe nonsense. The number of Americans who believe the Moon landings were faked is a widely quoted example. More recently we have the QAnon conspiracy theory, where 17% of Americans believe “a group of Satan-worshiping elites who run a child sex ring are trying to control our politics and media”. We laugh because it’s assumed that the British are far too level headed to fall for such nonsense.
This illusion is often sustained by the lack of impact most conspiracy theories have on UK politics. The British don’t have an MP who is mad enough to promote conspiracy theories, as Republican Representative Marjorie Taylor Greene has done for QAnon ideas. Further evidence might be the acceptance from the Conservative party leadership, unlike Republican leadership, that man-made climate change is a real threat that requires policy action, although this is a difference in what is said more than what is done.
To see this is an illusion I suggest starting with this post from Chris Grey. It’s about the delays many UK tourists are experiencing trying to go to France. The evidence that Brexit is largely to blame for this is overwhelming, yet Chris notes “the speed with which patently nonsensical arguments about it have been spread, and the sheer bone-headed, brazen obtuseness with which they are clung to despite every effort to correct them.” 
This is not an isolated example. Just as the problems caused by Brexit are many, so the list of false claims promoted to make people believe that these problems have nothing to do with Brexit are numerous. These false claims often originate in Brexit supporting newspapers but they are invariably repeated by government ministers.
Most if not all of these problems caused by Brexit were foreseen and discussed by opponents of Brexit during the 2016 referendum, but at that time were rubbished by the Brexit side under the collective heading of ‘Project Fear’. In most cases what was then claimed as fear is now fact, and the general line taken by those on the Brexit side is that these facts are either not facts at all, or that these facts have some other cause that has nothing to do with Brexit. The most common generic claim is that Brexit problems are really just acts of revenge by EU governments who are spiteful that we left.
During the referendum campaign the use of the label ‘project fear’ was extremely successful as a way of dismissing expertise about what would happen after Brexit. But when these expert predictions largely turn out to be correct, continuing denial becomes something even more alarming. As Chris points out, it didn’t need to be this way. It would be quite possible to admit the existence of trade-offs. Brexiters could say ‘yes, Brexit has caused problems, but it also has advantages that outweigh those problems’. So why have most Brexiters decided not to talk about trade-offs, but instead pretend all of Brexit’s problems have nothing to do with Brexit (what I will call Brexit harm denial)?
One argument could be that project fear requires subsequent denial, given the number of Brexiters who denied these problems would occur during the referendum. This argument is unconvincing, because one of the surprising aspects of Brexit has been how little the mainstream media have used previous statements by Brexiters as evidence against what they say now. Being proved seriously wrong in the past seems to be no barrier to the broadcast media allowing the same people to pontificate about subsequent events. Right wing newspapers think nothing of publishing mutually contradictory headlines within weeks, let alone years.
A better explanation for Brexit harm denial is that using the trade-off argument will not cut much ice, once it becomes clear that the benefits of ‘taking back control’ are either close to an empty set, or instead involve doing things that are generally unpopular. In a period where many people are finding it difficult to pay their bills, abstract notions like sovereignty cut less ice.
However I suspect even this explanation does not get to the heart of why Brexiters continue to deny the reality of Brexit harm. Instead we need to recognise Brexit as primarily a populist project, and ask why populists tend to routinely lie. Populism aims to ascribe general feelings of discontent and powerlessness to one or a small number of causes (immigration, the EU, globalisation, liberal social norms etc), and in addition to assert that these causes persist because they benefit a ruling elite. Populists like to draw a dividing line between their supporters (who are true nationalists) and the rest of the population, who are alien in some way (e.g 'woke').
Generally that association between feelings of discontent and the claimed causes is either greatly exaggerated or incredible, and so the only way of convincing people otherwise is by lying. As long as there is a large enough proportion of the population that takes little interest in politics and understands it poorly, lying can be successful. We are programmed to overrate personal confidence, so the populist never shows any doubt, or weakness, or fallibility. Lying becomes bullshitting in the Frankfurt sense. The more confident the populist appears, the more they appear on the side of those to whom they are trying to appeal to.
Brexit appealed to those who felt left behind by advancing social liberalism (see the appendix here), and the locus of that discontent was immigration. EU immigration was blamed for reduced access to public services and low real wages, claims that were greatly exaggerated or incredible and so required the techniques of propaganda or populist bullshitters to successfully persuade enough people of their validity. As real wages continue to stagnate and access to public services has got worse since Brexit, the only hope Brexiters have is to continue in the same manner by denying reality and ascribing any Brexit problem to something other than Brexit. Brexit populists need to appear to be still on the side of their supporters.
Which prompts the question: what difference is there between the way Brexit is currently defended and conspiracy theories? Both deny reality, or ascribe events to incredible causes. Both stress the need to believe in their cause, and dismiss experts as involved in a self-interested conspiracy to dispel their belief. Every problem caused by Brexit is claimed to be part of a conspiracy by the EU to hurt the UK or by ‘Remoaners’ to smear Brexit. Both Brexit and many conspiracy theories have at their heart some emotional feeling, like lack of control or fear of the other (where other can include vaccines). Both are surprisingly resistant to evidence. (Surprising to anyone who understands how science works). In this light it is no coincidence that those who support Brexit are more likely to believe conspiracy theories, and that populists often attempt to appeal to common conspiracy beliefs. (For a more academic take on links between populism and conspiracy belief, see here for example.)
Once we see Brexit as akin to something like QAnon, then any smugness the UK might feel about conspiracy theories in other countries evaporates.  Brexit as a reality-denying faith has captured the UK’s party of government.  Unfortunately, unlike some conspiracy theories like those about moon landings, Brexit has done great harm, and its proponents seem intent to increase that harm by breaking the treaty they recently signed.
Does seeing Brexit harm denial as similar to a conspiracy theory change anything?  Let’s start with those who supply the misinformation. They are not going to give up anytime soon, and their influence on public debate should not be underestimated. For that reason alone it remains important to combat the lies with the truth, because that is one way of preventing more becoming true believers.
But true believers in conspiracy theories are generally unpersuaded by evidence, which is one reason why so many accept Brexit harm denial. (As footnote  points out, over a third think Brexit has had a positive impact on the UK). Ridicule is counter-productive, and calling true believers stupid even more so. The starting point with combating conspiracy theories is empathy with the underlying concerns that motivate the false belief. If I’m right that a key underlying concern motivating support for Brexit was a reaction to liberal social change and in particular immigration, then this is very hard for social liberals to do, particularly in the face of policies like trafficking refugees to Rwanda. 
The end point is that it will be both hard and will take a long time to significantly reduce the proportion of the population that accept Brexit harm denial. Because of our FPTP voting system that proportion will have an oversized influence on at least the next election. This realisation may be one reason why a recent poll showed 51% believing Starmer was right to commit Labour not to rejoin the Single Market or Customs Union, and only 24% disagreeing. That should never mean Labour joining in Brexit harm denial, but when a combination of the government, much of the media and over a third of the population accept what is akin to a conspiracy theory, those of us who like evidence led policy have to be realistic about our current predicament and recognise the importance of getting back a government that deals with reality rather than its own fantasy.
 The main cause of these delays are the requirement that tourists entering the EU from third countries, which the UK is after Brexit, to have their passports stamped. Before Brexit tourists could just wave their EU passport at an official. Multiply that small extra time taken by the number of tourists crossing the Channel between England and France in summer at a few specific points and you get long delays. Chris outlines the spurious stories that Conservative newspapers and ministers (including the two candidates to succeed Johnson as Prime Minister) have promoted in an effort to suggest that these delays have nothing to do with Brexit.
 Like most other conspiracy theories, Brexit has a minority appeal, albeit a sizable and currently very powerful minority. In July this year, 48% thought that Brexit had had a negative impact on the country, while 37% thought it had had a positive impact. (Less think the government is handling Brexit well, but this comparison shows that many ascribe Brexit problems to poor implementation rather than reflecting badly on their concept of Brexit.)
 If you suspect, following their leadership contest, that Brexit is not the only example where Conservatives are living in an imaginary world divorced from reality you may well be right.
 Looking retrospectively, it helps explain the strength of the second referendum movement. Arguing that this movement was very unlikely to succeed, or that its leaders had mixed motives, is really beside the point. Perhaps an analogy would help. Suppose Johnson had declared that Covid was a myth and refused to do anything about the pandemic. Would it have made sense to argue that he had the power and there was nothing people could do about it. Of course not. Suppose he had put that policy to a referendum and won it, would that make any difference. Of course not.
 This helps explain why Cameron was the worst possible person to defend EU membership. Not only did he find it difficult to empathise with low real wages or reduced access to public services because he had run the country for six years, not only had he participated in the lies that this was due to excessive immigration, but he had also made a point of ‘modernising’ the Tory image by championing issues like gay marriage.