To provide clarity, this isn't (yet) (publicly) hooked up to the whole CCTV network of London Underground stations. Instead they use these temporary portable stations you can see at the end of the linked article.
I went to Victoria station where they were doing this yesterday and the police seemed quite unhappy with me taking photos of them operating the facial recognition station and confronted me almost immediately. The irony...
The area they were doing it in was completely swarming with far more police officers than usual, supposedly to "provide information".
I had expected to see signs all around the station warning passengers that this trial was taking place, but the only ones I saw were directly in that area or by the stairs as you're walking down into it. They weren't small enough that I could characterise them as intentionally hard-to-miss, but I reckon with the amount of traffic going down that staircase, the overwhelming majority of people would not have actually been aware that they were being subject to this LFR trial. I didn't see a single person other than myself pausing to read the signs.
Edit: There also wasn't a single sign inside the gateline where you'd tap out and walk into the area that the camera station was pointing at. I asked the police why this was, and they claimed it was because the cameras are only positioned to capture people entering the station, not exiting. Maybe that's true, as long as you don't turn around at any point? I'm not convinced.