Fair deal frank, you are right, London does provide airlines with the bulk of premium fare passengers but let's analyse why.
1. Historically the BOAC/BEA/BA monopoly as government run airlines badly skewed the way people travelled by only offering a few direct long haul and a slightly greater number of medium haul services from provincial airports whilst, by means of pool arrangements and bitterly opposing development of routes by other airlines, they stymied route development which would have occurred naturally. Thus it became accepted that, with few exceptions, one HAD to fly over London. Nowadays people are looking to their local airports or Amsterdam, Paris, Frankfurt and even Dubai to avoid London.
2. There is has been a very definite skew to marketing the UK by various agencies over the decades to bring people in by London then, in the case of tourists, send them round the milk run (London, Oxford, Stratford, Edinburgh, York and back to London). In the years I sat on various boards and agencies trying to bat for Manchester I, along with the likes of Birmingham, Glasgow etc., was banging my head against a brick wall. Visits in the 1980s to BTA offices abroad found literature on other areas of the UK away from the milk run gathering dust, unopened, in storerooms and reporting the facts back to BTA HQ was pointless. Again this has changed but the air links to cater for the change in tourism have not kept up and people are funnelled through London regardless.
3. There is still an attitude in airlines that people who travel on business are, in the main, based in the Home Counties. Again, the figures belie this. The problem is that implant and other travel agencies are pressured by big discounts to fly people from the provinces over London because seats on services that have been influenced to serve London by decades of the implementation of items one and two have to be filled.
The creaking infrastructure of airports in the South East and the poor service in terms of direct flights from the rest of the UK is regularly put down to potentially poor premium passenger loads from provincial airports. Not every provincial airport on a small island could offer the sort of loads any given route would need but a more balanced air transport policy over the years, with two or three provincial airports being allowed a bigger share of the cake, would not have the UK in the situation it is now in where, once again, London HAS to have more runways whilst there is capacity and potential elsewhere if only nearly seventy years of London centric thinking could be turned around,