Similarly, using a funeral service, public or not, to push your agenda is considered indecent. You have the right, yes, surely, but have some respect.
It's not permissible to arrest someone on the grounds of "lack of respect" or "making a fool of yourself".
Arrests have been made under section 5 of the Public Order Act, which applies to behaviour deemed likely to cause harassment, alarm or distress, or under some other law (I'm unclear which) for behaviour "likely to cause a reasonable person to suffer fear or alarm" [1].
It's heavy-handed nonsense.
[1] https://www.independent.co.uk/news/uk/crime/abolish-the-mona...
You’ll get arrested for doing the same thing in the United States at state funerals there.