In a previous job, part of our company were tax advisors. Not only to companies but also to (very, extremely in some cases) high net worth individuals.
When WFH became A Thing (because of Covid) we all had to have training on what to do if the police, they usually turn up with dogs, turn up at your house as part of a tax investigation.
And the number one recommendation was "say nothing" and the number 2 (instruction rather than) recommendation was "take facial recognition off all devices. If they ask for your pin you will MOST LIKELY forget it in the stress of the situation".
(they also said if you have a dog get it locked in a room before you open the door and keep shouting that you have a dog that you are securing it and not to shout because it's reactive. Because the police just shoot them if they make any move in their direction! i am not in UK)
back to the protestors. (I have family experience of similar because of where i live). Yes, they advise actually writing the number on your arm. Also not to have your phone with you unless really necessary and if you want to film, use an old one with no sim card and no apps. Also no ID card or other identification on your person. And "no comment" all the way. And yes, they had experienced lawyers ready to jump in for you.
Personally - I'm ok with chaining yourself to things to slow down their operation to make your point. I am 100% against any kind of violence, intimidation or threats to anyone, especially the public. I just think that alienates people from your cause.
I thought Usha was excellent, calm, good advice and being very clear to her client what she's up against.