Going back to the late 70's, early 80's, British Midland, then Brymon, operated NQY/LHR/NQY (direct flights) utilising a 50 seat HP7, certainly when Brymon were operating the HP7 they received loyalty from the Cornish locals and load factors were probably around 65-70%, meanwhile Brymon operated a Twotter PLH/LGW/PLH and, later, a DHC7 PLH/LHR/PLH.
When the HP7 became sold, to help finance the DHC7's purchase, Brymon combined the NQY route with the PLH DHC7, there was uproar from the Cornish locals, that I doubt has even been forgiven, that their direct (to or from) LHR service had been taken away from them and passengers voted with their feet.
Then on to the scene come BA, famous for taking LHR slots, they did it to Manx and they did it to Brymon, why the PLH/NQY London service, now PLH has something of a longer runway, has not reverted back to split services I don't know, Air SouthWest have had plenty of opportunity to offer the Cornish their direct London service, they didn't and FlyBE step in to the breach, who is wrong, the Cornish locals, FlyBE or Air SouthWest?
BUT .... LGW is not LHR, LGW simply does offer the connections that LHR offers, what might be needed is a codeshare agreement with the likes of AF/KL and a service to AMS or CDG offered whereas passengers can step off one aircraft and step on to another aircraft and without farting around between LGW and LHR.