Scandic,
What makes Jet2 different from most other LCCs is that the aircraft are theirs. Not ILFC, not anyone - theirs.
For them to buy new aircraft from Boeing or Airbus (I would assume the former - 737/787 combo) it would put £billions over their heads which I am sure they don't want. There will come a time when they will need to introduce NG aircraft but I think some of the aircraft currently in service will be bought - maybe U2s 73Gs.
What has happened to the XL 739ERs? Are they still parked up down south?