There needs to be definition of 'leased'. ALL of tui's Fleet are now leased, the last owned aircraft was BYAW which was sold earlier this year.
I strongly suspect that all of the wet/damp leases from the various ACMi operators will have been cancelled as have J2. Im guessing at a reasonable penalty (however the force majeure card may have been played).
Regarding the sunwing canadian aircraft, again doubtful if they'll be seen in the UK this year, but suspect seeing as though TUI own half of sunwing doubt there will be any penalty to cancelling these.
Regarding the 757 fleet, several of these were extended to remain in the fleet just for the summer so a distinct possibility these will exit pretty sharpish.
So I'd (optimistically) expect the majority of TUI's remaining own leased Fleet to remain operational towards the year end, including the 757/767.
The next big question is what do they do with the max order seeing as though the rumours are its nearing recertification........