The "Left" aren't doing better because 'left-wing' and 'pro-working class' no longer mean the same thing. The "Left" (which I assume by the op to be those operating on the fringes of the political spectrum) have the image of caring about middle class issues like the environment or foreign affairs/human rights abroad. None of these issues are economic based and none of them particularly help the British working classes, no matter how noble a cause they are (indeed a lot of the environmental issues could be seen as anti working class because they're the ones priced out by these policies). That's not to say the "left" don't care or support working class ideals, they just prioritise the middle class left wing issues above working class left wing issues (but pay lip service to them).
I think the working class feel alienated by these left wing parties that should primarily be campaigning to make working class lives better, but seem to put all their efforts into far away lands or into issues that don't help the working classes.
I'd also say that some of the left movements' attempt to hijack Islamic organisations to bolster their own support has backfired, and it is actually a lot of their groups that ended up being hijacked by Islamic interests instead, which hasn't particularly helped their image in the eyes of the working classes
exactly