The heritage bus
The most famous type of double decker bus was the Routemaster. It’s what people often think of when they think of the red bus.
These vehicles ran for a lot of years but finally had to be replaced. However, one route, the 15, does occasionally have one of these complete with bus conductor and you can use it in the ordinary way, as a passenger.
They run at bank holidays and some weekends, between Charing Cross station and Tower Hill, a very touristy route.
Do not board or leave the bus when it is moving, and only at designated stops.