Exactly groundhog
You drive to LHR for longhaul flights.
Many take the train to reading or paddington to get to LHR.
If a major player offered a PSO route it would be well utilized for onward and outward connections to and from Cornwall.
Now the dominant alliance at LHR is one world and the most logical carrier to operate a PSO on LHR-NQY for connections would be BA.
cs