Several posters over the past two years have highlighted the racism inherent in the Brexit vote. It would seem that they were correct all along and that "extreme views have gained ground in Britain since the vote"
We have seen some of those extreme views voiced on here

Try:
"Several posters over the past two years have highlighted their belief that there was racism inherent in the Brexit vote."
Several posters have also clearly stated an abhorrence of racism, while still believing that Brexit is the correct path for the UK.

That the "UN special rapporteur on racism", someone tasked with investigating racism, should find racism is hardly surprising. If it is your job to seek racism you are likely to be able to find it in any society.