Union Jack:-
the nav clearly saved the day.
Agreed! As to the runway, I have a vague feeling that it might have been one of the emergency diversion ones, ie double the width of a standard runway. If that were the case, and I'm far from certain, then it must have been the most northerly, RAF Carnaby. Well south of the border of course, but if you extend the track from Kevlavik/Reykjavik to Prestwick and allow for some understandable wandering it might just be possible. It might also explain the very low fuel state given that the nav would have calculated the possible transit of Northern England as well as Scotland before deciding that further descent was reasonable.
As you say, well done that man! The hills and mountains of the UK give testament to many others at that time that didn't fare so well.