AA in the USA defers any 737 MAX return to service in its schedules until at least mid April, and this is still only an anticipation -
This infers that any UK re-certification would happen after then, as both the CAA and EASA have to approve the type and lift the grounding, plus TUI will have to commence pilot training and make flight training and flight manual revisions.
The implications of this IMHO are that this will be too late for TUI to fit their MAX-8 fleet into the Summer 2020 flying program, nor likely take any deliveries of the larger MAX -10 due in 2020.
So back to expensive short term ACMI leases and to retain the 9 757's?
This is just my Tuppence but I think is a realistic scenario.