Well, historically, the only route NQY had was LHR, besides ISC and a once weekly ABZ that is, so if anybody in Cornwall wanted to fly anywhere then it had to be LHR and the route was viable.
Since then they've started other routes from NQY whilst the LHR (connections) route became dropped, I'd like to bet that a lot of the pax on the NQY/MAN/NQY route are connecting on to further destinations via MAN, LGW didn't work because it doesn't offer so many connections.
But what is needed is a codeshare route whereas they can book, let's say, NQY/HKG/NQY with the one carrier.