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 )

Although the London Transport Museum is fun (some years back I decided to spend a week in my own capital city as a tourist, staying in a hotel in the centre, all day looking at stuff with tourists - and LTM is one of the things I decided to do) like most museums it does have a lot of stuff it can't display - but unlike most museums those things are sometimes huge like a bunch of buses, so they're not in a back room they're an entire other site, in Acton IIRC, the Depot, which is in fact open this weekend: https://www.ltmuseum.co.uk/whats-on/depot-open-days

Is Brooklands the place where the corner at the Silverstone racing circuit is named after? It's also known for its aviation history of course.