Only Met services via the local lines are able to access the Watford Branch from Harrow. Thus all Met services to Watford from London run via the locals. All Chiltern services run via the Met Main lines.
Significantly simplified diagram of the Metropolitan between Harrow and Rickmansworth:
As Metrailway says, there's no pointwork for the move you're suggesting. Has there ever been a fast service from Watford though I wonder?
Anyway, for future reference I can heartily recommend carto.metro