If Jet2 did a London airport it would no doubt be Gatwick since that's the London airport which specialises for leisure traffic but can't see that happening just yet. Personally I'd prefer Stansted for them if they were to go to London. I hope they don't do Liverpool since EasyJet and Ryanair already have a big presence there, LPL need to look for non LCC's and Jet2 already have LBA, MAN and BLK, which are probably better for them anyway. As for Birmingham, not really much of a need just now since Monarch has recently increased alot there and plus they already have a growing base at East Midlands but could work out for them without damaging this.
I'd like to see Cardiff as their next base, making them the only low cost carrier to have bases in England, Scotland, Northern Ireland and Wales but Cardiff seems too far south for them. Would also be nice to see Prestwick but I think we all know that that's very unlikely to happen!