The 200

The 200 was a London double-decker bus transported by wormhole to the planet San Helios.

Profile
When the transit through the wormhole took place, the bus had eight passengers, including the Doctor, the fugitive Christina de Souza and its driver.

A small party of Tritivores, also visiting San Helios, though through conventional means, mistook the 200 for a weapon, though the Doctor disabused them of this notion. The bus was badly damaged and was stuck in the sand but the Doctor used an alien device to fly them back back through the wormhole (the bus driver having died) back to London, pursued all the while by the ravenous the Swarm. He had fitted the bus with anti-gravity clamps from the Tritovore ship to enable it to fly. Back on Earth the Lady de Souza took opportunity of a distraction (and, secretly, the Doctor's help) to steal the bus and escape in the flying bus. (DW: Planet of the Dead)

Behind the Scenes
The number 200 commemorated the two hundredth Doctor Who story aired on television, namely Planet of the Dead itself.