Best place to stay outside London travelling from The Midlands with easy train access into the city

[Frenzi]

Looking at going to the theatre in London with an overnight stay travelling from The Midlands.

The train fares are ridiculous so we have decided to drive so far, stay over night and get the train or tube into central London.

Does anyone have any ideas on somewhere to stay that means we have easy access into London but don't have to drive into the city?

Where in the midlands? Which motorway will you enter London on?

Watford? Several chain hotels right near station, trains to Euston then short hop on tube?

M1