The sign is used to indicate a stop line, not how traffic lights work. Often the sign might be a several metres in front of the traffic light to allow for the flow of oncoming traffic.
It's at temporary lights for roadworks, usually when a lane is closed. You need to know where to stop, which might be further back so incoming traffic can get past you
Crossing on red as a pedestrian is fairly common in the UK. The whole concept of jaywalking is not really a thing there and legally there’s nothing that keeps you from crossing a red light as a pedestrian. It’s merely a recommendation according to the Highway Code.
I’m German and it makes perfect sense to me. Mandatory traffic lights are ultimately car infrastructure and suck for pedestrians. Prohibiting to walk because there’s a light bulb somewhere is a very stupid concept. It works perfectly fine so why not elsewhere?
The only time you need to stop as a pedestrian in the UK is when instructed by police.