We need badge-equivalent cameras. What I mean is that we need to make cameras work the same way as badges and uniforms, in the sense that you just can't do police work if you don't have a working camera. A working camera has a red light on it which means that it is currently streaming (possibly buffering if temporarily out of wireless range).
The police don't have to record anything, they just need to stream somewhere with associated meta data. NGOs will come forward to read the streams and save them.
I understand it will be hard to get unions to agree to this and I don't have a solution, but I'm minded of the reforms made to policing in Northern Ireland as a success story.