I'd have a serious rejig of policing not just in London, but across England and Wales (Northern Ireland has it's own unique factors that mean it should be considered separately. Scottish policing is ultimately under the control of Holyrood and, whilst I don't like having one centralised police force like Police Scotland, it's up to them to do what they want).

First, I'd abolish Police and Crime Commissioners (PCCs). They don't increase the public accountability of the police and just use up money. Second, I'd abolish multi-county police forces (e.g. West Mercia, Thames Valley, Devon and Cornwall, etc.). Each 'rural' ceremonial county gets its own local police (known provisionally as a County Constabulary). These constabularies would be in charge of policing the towns, villages and minor roads of their county. Each of these forces would be the responsibility of a committee consisting of 10-20 Cllrs from the Council area(s) that Constabulary polices. They'd be in charge of enforcing standards and can appoint/dismiss senior ranks.

Metropolitan counties (Greater London, Merseyside, Greater Manchester, West Yorkshire, South Yorkshire, Tyne and Wear, West Midlands) would have their own 'Police Forces' similar to what most have now, but the mayor would have practically no control of the police who would instead, like their rural counterparts, be subject to committees of Cllrs (or possibly AMs in London).

Finally, I'd remove the non-London duties performed by the Met, abolish the British Transport Police and the National Crime Agency, and give all their duties to a new UK-wide force called something suitably iconic (like the FBI, Mounties or Federales) to be named at a later date. They'd be accountable directly to the Home Secretary, and would be in charge of policing all major means of cross-county transport (motorways, trains, airports, etc.), provide protection to political figures and royals, as well as investigate crimes like bank robbery, county lines, human trafficking, gang killings, basically all the really serious, organised stuff).