Yes, you
can find Marxists in the UK. That does not at all change what I wrote. The narrative of Marxism as some big scary bogeyman in UK politics is an alt-right trope. Suggesting the SNP, which is only moderately left wing even by UK standards, is close to Marxism-Leninism is equally ludicrous.
Put another way: If Marxism had any prominent support in the UK, RLB would have been outflanked on the left in the Labour leadership elections, rather than losing to someone on her right.