HZ123 -
The reason for the popularity of the London area airports is simple. That's where most people live, it's where foreigners want to visit, and it's where the money is.
Look at the population living within 40 miles of Luton. Compare that with a similar radius around Gloucester, Bristol, Cardiff, Southampton, East Midlands, Newcastle and even Manchester. Foreign visitors want to go to London, they don't know much about the other cities, even though we know they are wonderful.
The LCC destinations from the provinces are predominantly leisure-orientated, and are therefore stealing seat-only business from charter airlines. From London, other routes are busy enough to support a choice of airlines (eg I think FR fly 7 times a day from STN to Rome, and that's in competition with EZY, BA and AZ). There is also a large enough and wealthy enough market for FR to fly to Rodez, Valladolid, Brescia etc and fill their aircraft. How well would (say) Jet2 do if flying LBA-Carcassonne?!?