Your challenge is to have a left wing party advocating for English independence as a major force in contemporary UK politics, regularly polling at least around 10% in general elections.
By left wing, I mean in the sense of being economically interventionist (this can mean anything from Social Liberalism to outright Communism), but whilst I'll allow it to be somewhat socially conservative (like the SDLP), it can't be outright ethnically nationalist, so no Front National equivalents.
Bonus points for a post WW2 PoD.