Britannia Airways Boeing 737 on runway

A holiday snap shot of a Britannia Airways Boeing 737 sitting on a runway somewhere with another not too far away. This seems to have been taken from the window of another Britannia aircraft because in the foreground is a Britannia transit van parked next to the embarkation stairs of the plane the photographer is on.

Not sure which airport this is, but the fact that there are so many Britannia aircraft in the picture (there are more in the background) I assume it is a UK one. As for the date, it seems that Britannia started using the 737 in 1968, so this picture is after that date; because the picture was taken on Instamatic film which was most popular in the 1960s to 1970s, it is probably early 1970s.