Oddly enough, we had a day like that when I was custody officer. A Saturday - which was surprisingly quiet on a 3- 11 shift. Only 3 in custody. The shift Inspector arrived at about 4 pm and said, in a very excited tone, 'No wind up, there has been a murder at Newmarket Races and there is a coach load of suspects coming in'. There were 49 of them, and it became a very busy evening/night. With loads of overtime!I would Imagine it hinges on any application to march under the Public Order Act. Police must balance any lawful protest.
Problem is when people go in excess of the agreed protest (or break the no-surprises policy).
And frankly, I would be surprised if Cambridgeshire Constabulary could cope with cell space for 20 from one incident, and write off 80 officers (say 4 per detainee to arrest and remove).
I am merely commenting from the outside though.
Newmarket, despite being in Suffolk, one of the race courses is in Cambs.