'NO platform for racists and fascists' doesn't mean that they can't organise or say what they want.....and others are free to counteract them, shout them down or whatever,...thats a matter of tactical preferences.
It means not affording them a platform, i.e inviting them on yo your own or others'platforms, and campaigning to stop public institutions offering platforms to those who use them to intimidate and harrass part of the community.
There are many things that are legal, but are wrong. legislation always falls behind shifts in public opinion and tolearance.
Remeber Cable Street.