What do they do with all that video?

Dump it out of a buffer? Or store it, and process it with number plate and face recognition software?

Living in UK (bafflingly high number of CCTV) cameras makes me gently wary of this kind of surveillance.

Having said that I'm in favour of anything that helps prevent abuse by police officers. I used to trust police but recent scandals in UK prove that corruption and blatantly illegal practices are common.