People may want to travel to Cornwall Tommy?
The word there being "may"!
A NQY, then PLH, route survived fror umpteen years, operated by BMA then Brymon then ASW and it's 'bread & butter' customers were the Cornwall/Devon community, the locals, and business people like/need a morning and evening service five and two half days per week and this worked for umpteen years!
But, it seems, you want to operate a service for the tourists, the holidaymakers, those that might only travel to Devon/Cornwall once or twice per year, indeed per lifetime, only during the short summers and with big shiny jets and at low cost. Have you ever occupied significant time in Cornwall during the winters? I have, during the summers it is beautiful, during the winters it has to be one of the most damp and depressing places I have ever been!
Well Ryanair couldn't make this work with B737-800's, have you seen EZY jumping in there with their somewhat smaller shiny jets, even FlyBE with their regional jet/turboprop fleet? Nope, it seems they all realise that a loco operation to/from Cornwall is not what the market is crying out for. Do you see big shiny jets operating between the London airports and Blackpool, people MAY actually want to go to Blackpool you know?
As for connections via SEN, well NQY already has a MAN route that offers more connections than SEN is ever likely to offer.
But it seems to me there is someone, around here, living in Kent who wants to travel to/from Cornwall but isn't willing to pay a reasonable air fare to do so. Cornwall/London/Cornwall is, and always has been, a commuter aircraft route, a maximum of 50 seats but preferably less.