Agreed I could see Jet2 in LGW being a good fit, but I don't think they'd drop STN for it, certainly not entirely. One of the opportunities they saw with STN, apart from a wider deal with MAG, was that the package holiday product was severely underserved in that market.
They've done a great job in building a valued alternative to just Ryanair on a lot of the popular leisure routes. They've invested an awful lot in developing their brand in the STN and BHX bases very aggresively over the last three years, not to mention they also have their own ground handling and passenger services staff in-house so a move would be more complex. In my view they'd be daft to undo that at the drop of a hat to musle in on LGW where TUI and EasyJet already have that market well covered for now.