Originally Posted by
corporate-wage-slave
London may be offering that ticket in the H bucket, GLA in the O bucket. Perfectly possible.
If that's the case then there may well be mileage (literally in terms of bonus Avios, but that's not what I mean) in booking a holiday with a rental car or hotel to see if that shifts the ex-LHR fare into a cheaper bucket, saving you the need for a one-way to GLA.