Some strong evidence for Radlett here - but he's passed on..
Unmutual Prisoner Locations Guide (McGoohan/Portmeirion)
Actually I took those photos seen on that website in 1973. The problem with Radlett is that the woods which can be seen in the opening sequence do not appear to match the tree line at Radlett but do match the tree line at Bovingdon.
If it was taken at Radlett then the camera must have been at the 03 end as there is a railway running parallel to the runway. If that is the case where is the control tower which would be to the left and why is there a building to the right when there should be no buildings but a railway line?
Look carefully at post 7 and now look at the link in the original post. By using the two woods seen to the left of the runway in both pictures as a reference, tell me it is Radlett as I say it is Bovingdon.