Those who are devoted to Brexit have only faced one real enemy: economics. It is in the DNA of economics that trade is good, and so anything that makes trade more difficult will be costly. On top of this basic insight on which so much of society is built is a host of detailed evidence on the impact of trade agreements and the effect of more trade on the economy. This knowledge had become received wisdom among most MPs, probably as much if not more through their contacts with business.
What Brexit advocates had on their side was the huge advantage of fanatical support from most of the Tabloid press as well as the most widely read broadsheet . It is a schoolboy error to say that because newspaper circulation is declining its influence is also declining. More people may get their news online, but newspapers remain a primary news source online, either directly or indirectly. The broadcast media also tends to take their lead from newspapers. This is why Leave wanted a referendum, because they thought they could win, not because they had any great belief in this form of democracy. 
In the referendum itself economists were largely sidelined, and when their arguments did appear via the predictions of organisations like the IMF they were generally accompanied by a matching segment from the tiny band of “economists for Brexit”. For this and other reasons I have discussed at length in earlier posts, the Brexit advocates won, narrowly. But the advocates of Brexit still face a problem. If the news as Brexit happens is all bad, then maybe people will begin to hear about what the overwhelming majority of economists have been saying.
And sure enough, that has been happening. Sterling crashes the moment markets hear about the vote, and this will gradually reduce consumers’ real incomes. The Bank has to cut rates to their lower bound and restart QE. Forecasts of the public finances deteriorate, most recently here. So they needed some way of smearing all these negative medium term economic predictions. When the Bank of England revised up their forecast of UK GDP growth in 2017 from 0.8% in August to 2% in February (see this post), they saw their chance.
They will now claim that economists are completely discredited because they all thought GDP would collapse as a result of the Brexit vote and it hasn’t. Therefore anything they say about the public finances or growth in the medium term can be discounted. Now of course among those that know anything about economics this is nonsense. But most people do not know much about economics, they do not read the FT or Economist, so this kind of propaganda is effective. Don’t be surprised to hear it from political journalists in the broadcast media before long.
So for the record, before this happens, here is why it is nonsense.
- (And most important) Short term unconditional macroeconomic forecasts are extremely unreliable: always have been and I suspect always will be. They are slightly better than guesswork, but for a central bank that slightly better is well worth having. Predictions about the long term impact of Brexit mainly come from the non-macro part of international trade: gravity equations and all that. Their empirical foundation is strong. The idea that lower immigration will hurt the public finances is also common sense once you recognise that immigrants are young, and therefore will pay taxes that finance their use of the NHS and other public services with plenty to spare. This has nothing to do with macro forecasting!
Actually it is not the case that economists universally thought GDP would collapse. Here is the FT survey: most thought it would collapse, but it was not universal. The FT survey focuses on City economists, not academic economists. One prominent academic economists, Paul Krugman, has always been very dubious about talk of a recession. What is true is that economists universally think Brexit will have bad long term effects.
As I wrote here in June last year, the macro impact of Brexit involves counteracting forces. The depreciation is partly in anticipation of the loss in trade Brexit will bring, but in the short term it could boost net trade. Consumers could beat (for now) the increase in the prices of imported goods by buying durables immediately. Most forecasters thought these effects would be counteracted by negative effects, and it was reasonable to do so, but a sharp downturn was never a one way bet. In contrast, there is no upside to making trade more difficult with your nearest neighbour. The only question is how bad will it be.
If you think this is all so obvious that the propaganda about economists being hopelessly discredited will not work, I think you need to get out more. The line 'they all got the immediate impact completely wrong so we cannot take their medium term predictions seriously, and who can forecast until 2030 anyway' will be repeated ad nauseam in the press and by Leave advocates. Most political journalists will not know this line is rubbish and full of elementary confusions, because they do not talk to academic economists either directly or indirectly. The one group who could puncture this bubble is business, but at present its voice has been fragmented and therefore weak.
 For those who still doubt the power of the tabloid media, imagine there is a car market with a single best selling car, call it car R. A new rival is launched, car L. It is independently assessed to perform worse, but this assessment is not widely available. But car L gets a year of pre-launch publicity in 80% of the tabloid press, followed by a period of 6 months non-stop adverts for this car coupled with stories about the failings of car R. All discussion of the technical merits of the two cars in the broadcast media involve debates between advertisers for both cars. Now, honestly, are you going to tell me that under these circumstances car L would not capture a lot, perhaps a majority, of the market?