There were and are obviously limitations to what can operate and I remember some NW flights had to fuel-stop on-route. However, I don't think it was regular. In terms of the current regular long-haul operations from GLA the runway doesn't seem to cause any problems - I've haven't heard of flights having to refuel any time recently. If it was as significant a problem as some people seem to be attempting to suggest then surely the airlines in question would have moved to PIK long ago.
The argument about 747s seems slightly beside the point - I can't see a regular long-haul scheduled service being busy enough to justify a 747, whether it was operating from GLA or PIK. This is rather shown by the fact that current long-haul ops from GLA are operated by 767/A330, which have capacities far more suited to the routes being operated.
As is stated earlier in the thread, until about 1990 PIK was designated as Scotland tx "gateway" and as a result all tx flights up to that time had to land at PIK - whether or not GLA could handle them was irrelevant as they were legally obliged to use PIK no matter what.