Thomson also allocated seats to pax who have not paid for meals, groups travelling with infants and children, so you may find even if you get there early you might not have alot to choose from.
As WATABENCH said advise the check in agent that you have a nervous pax and im sure they will do there best to help.
You may be able to buy extra leg room aslong as these are not located next to the emergency exits doors as Thomson will not sit nervous pax right next to the doors.
PC