bus, usually red, with seats on two levels connected by internal stairs. On some tourist buses the upper deck is open, but all public transport buses are completely closed (it rains a lot in London).
On older London buses passengers board by stepping onto an open platform at the back of the bus. They then have the choice of going upstairs or staying downstairs: at some point a bus conductor will come along to sell them a ticket. Probably the most successful bus of this design was the Routemaster.
Newer buses (the majority, unfortunately) have doors at the front for boarding, and doors in the middle for alighting. The doors are operated by the driver, who also collects fares from passengers as they enter. These buses, therefore, usually spend an inordinate amount of time at bus stops whenever more than two people get on at once; and when the bus is stuck in heavy traffic only a hundred yards from your stop you just have to sit and wait (unless you have the nerve to push the 'emergency door open' button). This, apparently, is 'progress'.
A double-decker is also a chocolate bar made by Cadbury's: it comes in a purple and orange wrapper and has a crispy layer and a chewy layer. Sounds horrible.