Apologies for slight pedantry, but the bulks of the models are predicting that we'll be "relatively worse" by 2030 (i.e. lower GDP than if we'd stayed in the EU, but still higher overall). And I'd suggest having a look at the OpenEurope model forecast. They're a mainstream organisation, frequently cited (so not nutters). They've actually attempted to model the UK implementing trade deals with the rest of the world, which others haven't - a valid criticism.
Ultimately though, I don't believe anyone has tried an economic detangling of this scale before - the models are likely to be off the mark no matter which way you look.
As I understand it (and I'm entirely happy to be corrected), the rules explicitly mean that EU members can't enter into an agreement with the UK. Formal talks around a deal which takes affect after Brexit could be permitted, if the EU were willing to entertain them. It's just a matter of interpretation.
Yes you are correct, I should have made that more clear. The majority of models are from the baseline position. This type of modelling is actually more accurate than an attempt to "absolutely" model the economy as it cancels out the larger global shocks (say a major downturn from China or a civil war in Turkey).
The openeurope report you cite has a sequel which puts an estimated long term 0.5 to 1.5% loss of GDP. Given uk growth has hovered around the 1-2.5% mark for a while, this could mean being tipped back into recession or at least a significant drag.
It is also important to note how it models that the UK can offset this loss.
1. Trade. It points out how vital keeping free trade with the EU is. It also points to unilateral trade liberalisation and notes that this may have negative effects on some industries, in particular farming.
2. Immigration, it notes that in order to keep the economy going high immigration rates will be required.
3. Regulation. Here's a pertinent quote
We estimate a politically feasible deregulation agenda could lead to permanent gains of 0.7% of GDP – with savings coming mostly from three areas: social employment law, environment and climate change and financial services.
Right, because we don't like things like maternity leave, sick pay, clean water ways, clean air and soil, a stable climate and a sea level that stays put.
The financial services industry and banking sector have proven themselves perfectly able to responsibly self regulate and haven't been caught miss-selling PPI, card protection insurance, or fiddling the LIBOR, or nearly crashing the world economy and bankers are so poorly remunerated they can barely get by without multi million pound bonuses. So yeah, lets cut financial regulation.
As to trade talks with the EU, when we negotiate with the EU they negotiate as a block. It's one of the complaints Leave had about the EU. So when we have left, we will have to negotiate with the EU commission on behalf of all 27 states. We can lobby individual states but we can't make an agreement with them, it must be with all of them. So Greece might want the Elgin marbles back, Spain might want a chat about Gibraltar and there is nothing the German car makers (our white knights according to Leave) can do about it.