Labour will probably perform best in at the next election in places where there is very little 2015 UKIP vote to speak of (the Tories main pool of new voters). For example Labour would probably stand a chance at holding Hampstead and Kilburn (Tory target seat 11) even in a nationwide meltdown for example. Anywhere with large Green votes should deliver reasonable performances too, though unfortunately for them the Green vote is very low in most Con-Lab marginals.
You underestimate how much support has been lost as a result of the stance Corbyn has taken over Brexit. It's... um... bad.
What exactly is Corbyn's stance on Brexit? He doesn't really seem to have one.
