SE15 7GO was the postcode of the Powell Estate. (PROSE: Meet Rose) By another account, it was SU15 7GO. (TV: Army of Ghosts)
Behind the scenes
"SE15" is the postcode area of Peckham, one of the locations said to be the location of the Powell Estate, though this somewhat conflicts with the geography seen in several television stories, which should be Kennington. Either way, "7GO" is fictional, and "SU15 7GO" is even more fictional.