As Phileas puts states it has all been said before, only small sub 50 seat aircraft with frequency will work here and then we are back to the question of where will you land in London.
In the absence of any meaningful or imminent LHR expansion, NHT could be ideal with some infrastrusture improvement, especially for the 20-50 seater aircraft envisaged, and not just on the NQY route.
Clearly a SEN-type terminal-adjacant-to-station setup would be needed for the Central line tube and Chiltern railways. The latter could provide a 17 minute train journey to London-Marylebone.
Other London airports are unsuitable: LCY and LGW charge too much (hence BE's impending departure from the latter); LTN, SEN and STN are too far with slow and/or complicated rail access.
As for LHR.....let's not go there: tragically the best option (because of the connecting possibilities as well as point to point) is too expensive, too crowded, too prone to delays, and has slot acquisition problems.
Even if someone does get a London service of the ground again that does not solve Newquay's financial problems, time I feel to look at selling the place to private investors, people that understand the commercial World and save the local rate payers a lot of money.
Maybe not, but a London service would help a great deal, as is the case at any struggling small local airport.
How about going back to the old Dan Air Link City days and do NQY-DSA-PIK-MME-BOH-NQY and then the reverse the following day - that would keep these boards spinning for months!
Surely not DSA, it's relatively new as a civil airport, in the Danair days it was RAF Finningley.