It appears that having an “intention to cause alarm or distress” may be relevant for protests like this — arguably the entire point of the protest is to use the shock value of the nudity as a protest.

I was looking at this, too, it looks like public nudity is legal, and I don't think peaceful protest counts as disorderly conduct. The policing aid cites relevant law & provides a decision aid. It states

Naturists have a right to freedom of expression which only engages criminal law if they commit sexual offences or use disorderly behaviour that they intend to or are aware may be disorderly within the hearing or sight of a person likely to be caused harassment, alarm or distress.

A decision aid clarifies

Decision aid for responding officers

Is there a clear sexual motivation to the subject’s actions?
↓ No
Has the person been ‘disorderly’ and caused another person ‘actual’ harassment, alarm or distress (as opposed to considering the likelihood of this or the complainant finding it personally distasteful or offensive)?
↓ No
It will normally be appropriate to take no further action
Advise complainant

Personally distasteful or offensive doesn't qualify.

Disorderly conduct seems to mean disruptions that intimidate or prevent people from exercising their lawful rights or accessing goods & services they are legally entitled to. Police can impose restrictions on start & finish times, location, noise levels.

Maybe someone better versed in UK law can clarify.