We have a few such odd arrangements, such as the "London Bus Museum", which isn't in London but is in fact entirely within a completely different museum, the Brooklands museum.

Operationally independent, although they have been considerate enough to synchronise their opening hours.

If you're interested in London Buses however, I'd actually recommend the (also unrelated) London Transport Museum, as this one is located in the tourist heart of central London in Covent Garden.

( NB: Brooklands is itself a great museum, but more for the aviation history )