Re: Optional activation = propaganda
There are lots of answers to that.
The first is that victims have a right not to have someone stick a camera into their grief. If you don't accept that, go away and get some life experience.
Another article on here quotes "never store online what you wouldn't put in the newspaper" or words to that effect. You will lose loads of evidence if cameras are on full time, because people will just clam up.
And then there's the basic human rights. A PC has the right to go to the toilet without it being filmed and viewed by their boss.
Oh, and cameras are unbiased evidence? You don't really believe that do you? Centuries of camera trickery and Hollywood films should convince you otherwise.
Plus you've ignored the other point - anything recorded can be called as evidence, so you can't delete it. And if everything is recorded everything is disclosure, so every single case is going to take multiple times the amount of case preparation. Every interview with the victim, every interview with the witnesses all will have to be edited and anonymised.
Cameras on when outside by default, yes. But you need to leave the option to turn them off. If you want, add in a requirement to justify to control room why.
And factor in a minute or two to response times to detach camera from stab vest and attach to jacket every time an officer enters or leaves a building.
I'll get this in now - these will break just like any other piece of electronics that gets routinely smashed about and soaked in fluids. In 3 years most of the batteries will be on their last legs. You will see missing footage and it likely won't be due to any malicious intent.