On the face of it, If Jet2 offer a decent price for them, TUI would be mad not to let them have them.
But i like the football analogy Blakedean. If you were a manager, would you want to sell your players to a rival team? No, because they can provide an advantage over yourself.
In the airline case, giving your competitor extra seats to sell means, assuming 'Ceteris Paribus' they improve their Revenue/Profit at TUI (indirect) expense