Range of the a/c is one thing but longer required range requires more fuel which means higher Take Off Mass if everything else remains constant.
This is the sticking point, the ability to get weight off the ground. The Britannia 762s you mention out of shortish fields were almost certainly the 2 GTi aircraft. These have 763 engines on a 762 airframe so have performance to spare!
Those interested, they are G-BYAA and -AB with GE CF6 80C engines. The grand old girls BRIF and BRIG have the -80A engine. They are occasionally performance limited on long haul and are, well, old girls now.
I have no idea on theoretical ranges and only a vague idea of thrust for the 2 types.