Welcome onboard
Please post questions relating to Thomsonfly on the dedicated Thomsonfly thread. It just keeps everything in one place then so others who may have similar questions can find things easier.
If you've already booked extra legrooom seating, ie exit rows, then unless you want to sit on a specific row number, there's no need to book your specific seats. Extra legroom seats are limited to two rows in any case (15/16), positioned by the over-wing emergency exits. Unless you have a preference, there's no need to pay extra as they all offer a similar amount of room.
Darren