It's a technology. What you could do with it, and what you should do with it are completely different things.

If you follow the links there is a lot of information about the trial, including a whole series of documents identifying the legal framework that authorises LFR, including rationale under GDPR, Data Protection etc.

Of course, this technology could be used for bad purposes. But within a democracy, policing happens with the consent of the people, expressed through various channels including legal (parliament and local government) and police commissioners. What is interesting is the way that these frameworks are used to constrain the purpose and extent of this processing. For example, at the moment the images on the watchlist can only be drawn from custody images.

The police already have powers to seize CCTV and door-cam imagery in order to investigate crime. They need a warrant from a judge - so it isn't justified without good reason such as serious crimes. This is the way you constrain these things - you put the regulatory framework around them. You can't put the technology back in the box, so you put rules and expectations around how it is used.