I suppose that, simply put, if the airports didn't have the space then they couldn't be parked there. They would then be ferried to the nearest field with space appropriate to the aircraft's weight and size. It might also be that BA are an important/strategic customer of said airport and an extra effort is made to accommodate their plight.
PanAm had their own Mx facility at LHR, so at least there was some company space for their steed during the strike. I can remember the lovely cafe adjoining the facility and partook of a few deliciously unhealthy breakfasts back in the day :-)