Historically, in the days when a Handley Page Herald operated the NQY/LHR/NQY route average loads were between 30 and 40 pax.
The majority of these passengers would have been business travellers attending an office in London or travelling internationally, by means of a LHR connection, on business.
However, in this day and age there is the fax machine, the internet, email, cyber conferencing etc. etc. etc. thus more people are able to work from home than they could have done historically.
More people do travel in this day and age but moreso the loco leisure traveller, i.e. they'll only travel if the price is right.
Unless a Cornwall London route can operate to an airport where connections are available to recognised business destinations then the route would be dependant upon leisure travellers many of whom will not be willing to pay a fare that would support such a route.
I agree with Tommy that there is nothing appealing about STN, if the passengers want to connect then they'll spend circa 2 hours, in each direction, travelling between STN and, probably, LHR thus, many, will view that they're better off travelling from home to LHR direct or driving down the A30 to EXT to get a connection via CDG.
Alas BA 'swiped' NQY/PLH's LHR slots thus any LHR route is knocked on the head, LGW doesn't offer the connections that LHR does thus the route would only be partially successful and would ultimately fail, and as for LTN, STN, SEN etc, well pretty much leisure travellers only.
Any reasoning of holiday homes etc. falls by the wayside, those holiday home owners do not live in the catchment area of one particular airport, one particular London airport, those holiday home owners will be living far and wide from Scotland, Cumbria, Lancashire, Kent, Hampshire, Wales etc. etc. etc.