Interesting.
Normally, films use deliberately fictious numbers, e.g. in the US it's always xxx555xxxxxx. Wikipedia says the UK uses various area codes for the same thing, including 011x and 01x1. The Pink Floyd number is a bit unusual---it's not made up.
According to a previous analysis, the call was the album's "Chief Engineer James Guthrie who called his own London apartment", with a neighbour answering the phone. Someone probably knows roughly where James Guthrie lived in 1979/1980 and what the area code there was. But I don't.