I think there are some contradictions. There are some aspects of Islam that go completely against the views of the Left . I am surprised more on the left- particularly in student unions- don't speak up more about some aspects of Islamic groups' ideology.
I think there is also an irony in that the former Communist countries seem to be extremely intolerant to black/brown skins and immigrants generally.
In my experience some of the strongest anti immigrant views come in traditional lower working class areas but overall they still vote Labour rather than Tory/Liberal.
Finally, it is worth noting that not everyone on the LEft opposes Brexit. Many leading very left wing figures over the years, including Jeremy Corbyn, have been hostile to the EU.
Oh, and it's not just those on the Left who can spot racism and oppose it. I haven't voted for anyone but the Tories now since 1979. I dislike the sort of racism and strong anti Islam prejudices we read every day on AB ad nauseam