Most police officers work an official 12 hour shift, however they are expected to regularly forfeit their leave & extend shifts for urgent incidents in London such as demonstrations, new years eve etc. There is a new system where the MPS is trying to take on part time officers, this is a new idea which they are piloting now. I'm not a police officer so can't give an expert opinion on these answers.
You have to stay and keep your shift rota
This is probably not for you, policing is 24hours a day