The vast majority of letters addressed to 'top people' get opened by their PAs, who promptly redirect them to the same customer services team that handle all other correspondence from customers. (When I worked at a railway station we often got letters handed in that were addressed to the Managing Director. We were told that even if they were marked 'Private', 'Confidential', 'Personal' or 'For the attention of the addressee only' we were to send them all straight to our Customer Services department, so (unless Customer Services decided to forward them to the MD) they never got anywhere near to him. So you're probably wasting your time.
However Thomson Holidays is a trading name of TUI UK Ltd. Its Managing Director is Nick Longman and his correspondence address is TUI Travel House, Crawley Business Quarter, Fleming Way, Crawley, RH10 9QL.