With the LHR domestic debate it is worth considering that NQY-LGW was operating 2 daily and now 3 daily on a DH8 and isn't really that far behind in terms of passenger numbers compared to LBA-LHR which operates 3 daily on the A319.
Not that BA would, but If R3 was built and if they ran a 3 daily Airbus on LHR-NQY the numbers would no doubt increase based purely on the fact that a LHR link operated by BA or with a BA codeshare would offer connections onto the entire BA and OW network for both outgoing and incoming passengers.
Just jump on FGW at Plymouth and see how many customers are heading into Cornwall with bags tagged to LHR............
That's not to say that NQY-LHR would be a viable and profitable route, just that I feel that LHR-NQY would carry a higher passenger load compared to LGW-NQY
cs