The route is a LHR one historically and had a fair amount of transfer traffic. When Brymon were bought by BA, they nicked ["moved" surely - Ed] the slots for something with more seats and shuffled the British Regional DHC-8s off to Gatwick.
This ended up losing money and was eventually dropped. Fearful of losing the London link, (this predates Rynair's STN-NQY), Air Southwest were formed to pick up the reigns with some ex Brymon metal and there was no break in service. Air Southwest then expanded into LCY, which lost money, then latterly found the NQY leg of the LGW routes under assault from the airline that gobbled up the remains of Britsh Regional / BA CitiExpree / BA Connect, Jim French's resurgent flybe. Unable to sustain continuing losses, fares were hiked back up and we arrive at where we are today.
I have no doubt PLH/NQY would work at LHR, as would JER, INV and a few others. That would require a thought out transport policy instead of just chucking more A380s into LHR though.