I think there has always been a left wing case for saying that Actually, The EU Is Bad. For the UK specifically, however, I think this is at the moment quite wrong. First, the UK isn't under the thumb of the ECB which is, actually, bad, so they avoid that particular problem. Second, there is lefty stuff in the EU framework which is actually good. Third, Brexit isn't happening under a left wing government in the UK, it's happening under sociopathic Tories, and what they will put in place will enshrine everything (except the ECB) bad about the EU while getting rid everything good (free movement).
Brexit is bad for the UK at this moment, but there is a case that the EU is, as configured, bad.
