Protest should be legal across the board

You're floundering on the is/ought issue.

When you come in insisting that we shouldn't question the legality of waving a flag that says 'Kill The N-rs', you not only sound like you're championing bigotry but you miss the point of the spectacle entirely.

What's at play is a UK government that has empowered itself to proscribe certain types of speech and abstained from regulating other types of speech. The government's ability to regulate speech has already been ceded (even - perhaps especially - in the American surveillance state). What's notable is who ranking leadership uses that power to suppress and who it tacitly supports.

That said, I’m pretty sure “kill them all” is incitement to violence.

I would go a bit deeper and ask how many members of this protest group are members of or friends with the people charged with regulating their conduct. This isn't merely an incitement to violence. It is a platform of a party that is rapidly gaining support within the country.

What you're looking at is the seeds of an open insurrection, not unlike the J6 riots in the US, the Hinduvista riots in India, or the Israeli settler riots in the West Bank.