If you go to the LHR / NATs site
NATS | AIS - Home
you will see on the holding charts for 09L and 09R that at the time T5 was constructed taxiways were added to the western ends of the main runways. Officially these are not part of the main runways, which begin with the pedestrian crossing type markings immediately before the marked runway numbers.
There are at present no similar taxiway extensions to 27L and 27R, though I understand Heathrow (BAA) are planning such extensions after a rearrangement of the eastern surface car parks takes place.