Saying they were "just in an exclusion zone" is kind of a self-justifying excuse. It’s the same logic you see in authoritarian countries, like when Russian police arrest people for holding blank signs because they’re technically standing in a prohibited area.
If the government can simply label certain places off-limits and turn ordinary, non-violent behaviour into a crime, then the rule of law stops being a protection and starts being a way to selectively shut people up.
First of all, this is not, in any meaningful sense "the government" - the UK has an independent judiciary interpreting laws defined by parliament, but this was not at the behest of government in any reasonable sense.
Secondly, this is not the blanket labelling of a place as "off-limits" - it's off-limits to a specific group of people who have prior examples of harassing people in that location. It's no different in concept to a restraining order. A restraining order does not make the relevant locations unavailable to everyone, only those to whom the order applies, and the bar for one being granted is not, generally, negligible.
There are genuinely concerning cases where the right to protest has been curtailed (or is trying to be) in the UK at the moment. Some of the laws proposed around restriction of protest are illiberal and overreaching. This is not one of those instances though.
If the government can simply label certain places off-limits and turn ordinary, non-violent behaviour into a crime, then the rule of law stops being a protection and starts being a way to selectively shut people up.