For the record, a fully loaded i.e MTOW 744 cannot take-off from GLA. A flight to LAX from LHR ( minus the distance and fuel burned already travelling for an hour up to GLA in the diverted case mentioned) is still not at MTOW and therefore could struggle with any distances further than east coast / central US and would require ideal weather conditions on take off. I would imagine these factors would make regular operations less than reliable.
Field length for MTOW Boeing 744 > 3000m
Glasgow runway length Main Runway 05/23 is 2658m