EuroWings once served an airport in Essex, but I guess it depends upon who has the slots?
Virgin Express have new enemy especially adding Cologne to thier base list!
Gut feeling:-
1. Gatwick
2. Stansted
3. Luton
However, TBI, the operators of Luton are promising more 'good news' following the Baby base at Cardiff!
Low cost means just that, so the actual airport matters very little, I do hope they come to Luton, Charlie the Cologne bear is a good mate of mine!