'Height' is always a vertical distance above an airfield, the pressure datum is the QFE.
What you might be referring to is the Transition
Altitude which is measured above sea level using the QNH pressure datum.
It is currently 3000' in the UK, except for specific areas such as around London, Central Scotland and Belfast, where it is 6000'. In Manchester and Birmingham, it is something else yet again.
No idea about the Glasgow setting in the 70's, it has been at least 6000' since 1981 though, if not before
