The whole concept that allowing the free expression of objectionable ideas will allow them to be dissected and disproven in the cold light of public debate, has itself been disproven over and over for at least the past decade.
We live in a different era, where partisanship and propaganda are so powerful that there is no longer an authoritative voice that the general public trusts. The government is distrusted, the both-sides-BBC is distrusted, the broadsheets that turned into clickbait farms are distrusted.
We have to stop giving oxygen to fascists in the hope that people will just see how wrong they are. I don't care what people attending this march really believe. What I care about is that people preaching hate should not have a platform, anywhere, under any circumstances.