FWIW their employer is not the prosecution. UK police don't prosecute cases, the CPS (Crown Prosecution Service) does.

The word of a police officer, in UK law, is that of any other witness. There is a kind of presumption of regularity in the courts, but they don't have any sense of qualified immunity; they are generally but not universally considered not personally liable for negligence but that is not guaranteed them.

And unlike police departments in the USA they don't really have much latitude to experiment with technology. IMO they should be banned from using AI tools that aren't centrally provided.

Other than that, yes — I agree with your general view that this is an alarming state of affairs for people in a position of trust.