Surely it's not necessarily what he stands for but the violent way he does it?

Palestine Action acted as terrorists, attacking national defence infrastructure, attacking police with hammers, and called for genocide of Israelis.

You can protest in support of Palestinians without being violent, and without supporting specific proscribed groups; the state asserting it's monopoly on violence is normal. With democracy and rule of law - both strong in the UK - that's not something that will bother protestors.