I'm fairly certain the de Havilland Aircraft Museum, located on the northern outskirts of London, used to have one of those cannons. It wasn't mounted, but was on display with the loading mechanism and magazine.

I've checked their website[1] but can't seen any mention of it. The museum looks a bit more neat & tidy than I remember it looking, so perhaps they've reduced what's on display.

[1] https://www.dehavillandmuseum.co.uk/