AeroMad,
The punters that normally take flights normally take flights to, as near as damn it, their final destination, very few take a flight first to then connect with a train, a bus, a boat or whatever.
Don't get me wrong, having grown up in Croydon LGW is not only in my old stomping ground but my commercial aviation career commenced at LGW back in the 70's so, if anything, I've got a soft spot for the old place but LGW simply doesn't provide the connections LHR does so to/from LGW is would neccesitate a coach around the M25 wkith journey times often oimpossible to predict, the coach might be scheduled as 1hr 15mins but best allow 3hrs!
Pixie,
As with my previous career and soft spot for LGW in a previous life I also had a career at PLH, I have many fond memories of times at PLH not forgetting the early evening cricket matches whereas the old runway 06/24 served as the 4/6 boundary and the controller would wave from his pink control tower if we needed to stop hitting balls across the runway due to an inbound aircraft.
PLH/MAN should work because MAN is the UK's 2nd airport for business connections, punters can route PLH/MAN to connect with the mainstream carriers worldwide, far moreso than they can from LGW.
But STN ..... Nope, punters would be flying over London to reach Bishops Stortford Regional Airport, unless they are connecting with a LoCo they'd then need to catch a coach or a less than fast train in to London, then if they're continuing their journey by air they need to, invariably, get across to LHR by whatever means.
There is such a thing as the 'buggeration factor' whereas a punter calculates costs versus times etc. to/from PLH and LHR before deciding "Bugger it, I'll take a hire car, a train or a coach, it simply isn't worth all the buggering around".
A LON service needs to be to/from LHR but with slots literally impossible to come by then it's not going to happen.