When TOM flew to MCO with the 767's to guarantee a direct flight outbound it had to use either G-BYAA/AB which had the CF6-80C2 engines (hardly ideal from an operational standpoint) The Orlando market for charter carriers was stronger then as it was before VS & BA moved in with a lot of capacity.
The 787 can operate to plenty of long-haul destinations from a 7000ft runway. However, in the case of LGW, many airlines are now more interested in preserving the valuable slots they have there rather than moving aircraft elsewhere to operate a service from another London airport.