Great Portland Street was one of the original stations on the London Underground network. It was also the focus of an artificial rupture in the fabric of time and space caused by Thomas Brewster operating a time machine comparatively more primitive than the TARDIS. It allowed him to transport a train of people from Earth to Symbios.
In tracking down this time machine, the Sixth Doctor and Patricia Menzies landed in the original foundations of the station where they noted its old brickwork and its rats. (AUDIO: The Crimes of Thomas Brewster)