I agree that some kind of a recession is inevitable but going into a No-Deal scenario and going into a recession bordering on a depression benefits absolutely nobody except the small cabal of Brexiteers.
The leftist priority should be to protect public services and to facilitate job creation. Even with a Corbyn government, No Deal would be the worst scenario by those criteria.
No Labour policy is at all impeded by EU membership. The EU is a neoliberal institution by origin but nonetheless it is useful. Even as a Marxist, to me the point should always be to pick the progressive option, and right now the EU is the progressive option. Brexit can only by definition be a regressive step. Look around the world - where are the well-functioning social economies? Almost all in the EU, or extremely closely associated with it. If we were at a Denmark or Finland level, then I'd say it's time to start thinking about the constraints of the EU, but whilst we still have so much space to improve within the current framework, our priority should be reforming our internal problems and then looking outward.
I agree that the EU has done absolutely reprehensible things but even then, it doesn't always need to be the EU of Wolfgang Schauble. The EU merely gives expression to the consensus of the major nations - all it takes is a wave of serious social movements to emerge in Germany, Britain and France, and the nature of the EU would change substantially.