It seems that Government economists have been modelling their views of what may happen after Brexit. Surprise, surprise, it’s pretty much what economists were saying before the referendum.
Lets look at some facts:
Currently 43% of our exports go to the EU. This share is declining.
The other 57% goes outside the EU, and this share is rising.
Some argue that this is because of the growth in the USA and China that has increased everybody else's exports.
Assuming there will be at least some restrictions to EU trade post Brexit, we have to assume that our share of EU trade will reduce. We have no idea how much yet, as we have no idea what the outcome of negotiations will be.
What is also unclear is how much of the non-EU exports may be affected. These are currently covered by EU-wide deals, and we may, or may not be able to negotiate significantly better terms.
The big problem, for a long time, that we are not as good at exporting as we should be. And you just can’t blame that on the EU.
Look at Germany, for example. They were one of the founder members of the EU. They export about 84% of their GDP. (https://data.worldbank.org/indicator/NE.TRD.GNFS.ZS) The EU hasn’t held them back much, has it?
So any Brexit dividend has to assume either (a) we can make deals with countries outside the EU that are significantly better than we (and the EU) have at the moment or (b) our exporters suddenly figure out how to get a lot more business than they have done in the past.
In short, all the concrete things we can identify look as if they will suffer after Brexit, and we have nothing specific on the upside to compare. So of course any economic estimates will look gloomy.
But to say this is apparently being unpatriotic.