the UK happens to 1) not have a proper constitution with any encoded human rights and also poor judicial review and 2) an inherently authoritarian currently-ruling party and media ecosystem and culture 3) be a UN SC member, Five Eyes member, G7 member, reasonably big economy etc so it's government is in a good spot to push hard on this and see if it can break the will of tech companies, which would be a great technical and social precedent to help other governments achieve the same goal. Australia is also a battleground for this for similar cultural and political and legal reasons.
what is less clear is why Signal is saying this is a victory, unless they really are so cynical that saving face while losing is more important than standing up on this point of principle that their company was supposedly founded on.