Not that I want to start the old STN v LTN thing again but what really would Thomas Cook gain by partly moving to LTN? Part of the benefit of having 3 aircraft based is that they can offer more flexible durations and more destinations. It's not as though LTN is short of competition either (assuming Monarch stick around).
If I were Thomas Cook and had a spare plane laying around I'd make it 4 aircraft at STN and compete with Jet2 by adding some new destinations rather than put one into LTN.