@SorryImKnew
On what basis did the police advise Claire/Posey not to go ahead with the event if they feel within the exemption categories?
On the basis that we are currently dealing with on ongoing crisis caused by a pandemic. You mentioned this several times on the thread yourself.
During this crisis, any large gathering adds to the stress on the police. The pros and cons of holding political protests have been discussed up and down the country, in every country affected, both privately and publicly, by citizens, parties, civil rights organisations, the police and all levels of government.
Democratic countries have alighted upon an uneasy compromise that accepts the citizenry's right to free assembly in principle but limits it in practice through the use of tightly regulated exceptions (see earlier in the thread for those applicable in the UK).
The police is urging all organisers to take their protests online if possible, to make their jobs of policing the public's adherence to the Covid-based restrictions easier.
The police have however allowed various protests to go ahead unhindered, and have partially intervened in others (in addition to normal policing interventions of course). AFAIK this is the only political protest they have completely stopped.