Hi
Normally with a Thomson flight you pay to select your seats and are able to do so 90 days before the date of your return flight. You get an e mail saying the seating plan is available on line.
For 2019 we have booked for Skiathos with a TUI package deal, flying Thomson , and have elected to select our own seats. It was stated in the terms you can do so 90 days before the date of your return flight.
When we booked for Corfu with a Thomson flight, we have a 2019 TUI package holiday deal, there was no option to select your seats.
Selected the Norwegian Air flight option and were able to select your own seat option 90days before the return flight date as usual.
As Norwegian Air may now be possibly bought by BA not sure what will happen
Geoff