Ah, this is still on-going.
Suppose you were going to a point on the ground that was the same elevation as your departure point, it was not very far away, isobars nice and spread out that day in the immediate region, and there is no really close station to give you QNH at that point you are going to/by? - well what better could you use than the setting that gives you zero at your departure point? - the QFE?
Funnily enough, ZD576 had one subscale set to the QFE at Aldergrove and the HLS at waypoint A on the Mull had the same elevation
Oh and the RADALT idea wouldn't have been much good with a sudden rise up the cliff from the sea to that HLS.