Spawned by cheap coach line Megabus, it’s cheap and cheerful: as little as £1 when booked ahead. For that, you get a one-way, 2-hr. loop around town on an open-topped blue double-decker with three stops (Park Lane, the London Eye, and the Tower of London). Narration is recorded—one of the two voices is a Blue Badge guide—and triggered to play in the right spots by GPS. It even goes across Tower Bridge.