He says he was arrested under section 12 of the Terrorism Act 2000 which makes it illegal to stay anything even remotely non critical about a "proscribed organisation" like Hamas. It is extraordinarily repressive.
To clarify this is part of what section 12 says:
Proscription makes it a criminal offence to: express an opinion or belief that is supportive of a proscribed organisation, reckless as to whether a person to whom the expression is directed will be encouraged to support a proscribed organisation.
Only a handful of countries consider Hamas a terror group. Funnily enough these are the same countries that are complicit in Israel's genocide in Gaza.
This is a "leopards ate my face" law. It's fine until you get caught up in it however unlikely that appears now.