I am very sorry you were delayed and that this has obviously spoiled an enjoyable holiday. Sadly, I do not know the exact reason your flight was delayed but I do know problems with the long-haul program, two aircraft with engine problems and a weather diversion in the Caribbean did cause significant disruption to the flying program.
Thomson do maintain spare capacity, it has one 767, one 757 and one 737 as dedicated spare aircraft. Sometimes aircraft can be brought out of the maintenance program if time permits.
However, at times like this the operations department do work extremely hard to acheive the best outcome and the well-being of passengers is at the top of the list of considerations. If a long delay is foreseen the best option is always to keep the passengers at their accomodation if possible. But things do happen at short notice and not everything can be foreseen. Operations reacts the best it can. An aircraft charter will always be considered but is not always available or is not the best solution to the problem.
I know delays are frustrating and I am sorry you were not better informed of the reasons. However, Thomson does takes customer care seriously, you only have to look at how customers were treated during the volcanic ash disruption.