I'm hugely impressed by all of the many spots that members have mentioned in this thread - please keep them coming!
Here's a few more from me to add to those that I highlighted in my original post.
There's a couple of star appearances in the brilliant 1963 movie
The Spy Who Came in from the Cold. First up, there's RM2211, which appears on the 6A to Waterloo:
You can see a GIF of its appearance
here.
In the same film, RM2200 appears on the 207A to Whitehall:
A GIF of that bus's appearance is
here.
In the 1979 remake of
The Lady Vanished (which is set in 1939, weeks before the beginning of World War II), a couple of unidentified buses (or, more likely, the same bus with different blinds) can be seen. The 12 to Whitehall:
...and the 54 to Whitehall:
In the film, the protagonists must get to Whitehall to deliver an important message, which is presumably why the buses were shown with Whitehall blinds. But has 'Whitehall' ever actually been used as a terminating point on bus blinds? And if so, did they have a qualifying point as shown here?
Alas, it's impossible to see exactly what's printed on the blinds as the buses are only seen out of focus in the background.
In the 2013 BBC production
The Great Train Robbery, a single Routemaster appears in multiple scenes. First, in the opening title sequence of the first episode (featuring the gloriously contemporaneous sight of a conductor on the rear platform smoking a cigarette!):
A GIF of this blink-and-you'll-miss-it scene is available
here. Despite supposedly being set in London, the scene was quite obviously not recorded here - note the bus shelter and stop in the background!
The same bus appears in the second and final episode as the 53 to Piccadilly...
...including a completely pointless (from a narrative perspective) close-up of the side number-blind:
The same episode also includes something of an anachronism - despite being set in 1963, you might just be able to spot a blurry Enviro 400 visible through the rear window
in this scene!
Fast-forward a few decades, and you may have seen VW1189 make an appearance in ITV's
Heathrow: World's Busiest Airport while in service on the 105:
...alongside Metroline's VP48 on the 140 in the background:
Not so much a London bus, but a London bus
stop making an appearance in
Red Dwarf: Back to Earth...
...at the fictional 'Stelworth Heath', heading towards 'Lumpton Bus Station'.
And finally, this is most definitely not a London bus, but it surely deserves mention as perhaps the wildest bus ever seen on the silver screen:
It's from the 1976 spoof disaster movie
The Big Bus - a wonderfully silly film with a surprisingly star-studded cast. The large glass window at the front-right of the bus is where the onboard bar-lounge is located; the large glass upper deck dome structure towards the rear is where the swimming pool is situated; and
this GIF will give you a better look at the nuclear reactor that powers it (!!).