Key question is when was the ban actually imposed - It might have been later than 1981.
The ban applied to the Glasgow Central station area and approaches only, no issue with Peaks going to Shields Depot or Corkerhill, which they could access from Barrhead without having to go via Central, via the Terminus Curve.
I started in Glasgow Control in 1984 and can recall one occasion when Carlisle, presumably as nothing else was available, sent down a Peak on one of the morning trains; Locos were changed at Barrhead and the Peak did not get to Central! Quite possibly a similar loco change took place on a returning Up train, to get the Peak back south.