Critical Mass = Legal

Perhaps. But this was decided by Law Lords, so it means (if I'm correct) that it was decided in the highest court of appeals in the UK. And I believe most, if not all, Police services in England and Wales are subordinate to the Home Office... so this ruling against the Met might indeed have ramifications in other areas in England and Wales.

The police forces are local and controlled by local police authorities. The Met used to be under the aegis of the Home Office but now is under the control of the GLA police authority. (viz the dispute between former Commissioner Blair and the current Mayor)

The law laid down has implications for all E&W and probably Scotland and NI as well but the implications are considerably greater for London because no one really gives a toss about outside London.