"a few points off GDP" would mean even greater poverty and a existential crises for the NHS. To salvage some sort of socialist policies out of that you would have to have an old school, centrist, authoritarian regime - imposition of capital controls, seizing assess - basically an isolationist model that nobody is going to vote for. More likely its crises management and then a real danger of the tories getting back in with their eager axe. And a more isolationist UK will also be a further boon for the ugly naitonalism that brexit has released.
The here and now is not about building socialism through some convoluted set of circumstance post brexit (seriously - what are the chance of that? its fucking delusional) - its about keeping the NHS, reducing poverty and giving people decent homes and jobs - brexit will fuck the chances of that big time. precisely because of " a few points of GDP" (i.e. a major recession) followed by reduced economic growth and higher prices.
Yes - the EU as it is currently set up is still pushing for greater neo-liberalsim, but their is also greater resistance to that across the member states - especially post 2008. And those neo-liberal forces will be even harder to resist outside the EU - because the UK will be in a much weaker position and coronations and major economic powers like the USA and china will be pushing for much greater "liberalisation" in return for market access.
Scrap brexit. Stay in for now, do whats possible, see what happens.