At the end of the day, you are both paying for a seat on that aircraft, any seat. If you want the privalege of specific seats or seats together, you pay more.
The airlines job is to get you to your destination. If your happy to accept this and do it the cheapest way, you can opt out. You can skip meals, seats together, extra leg room, even hold baggage. All optional extras that tailor your flight, which will cost you.
Thomson, like most charter carriers, will seat "no meal" passengers towards the back of the aircraft anyway. So if your not having a meal, then chances are when you get to check-in your seats will already be allocated. Either a window and next to, or adjacent aisle seats (providing your on a 3-3 config aircraft short/mid haul)
If you have booked meals, then chances are your seats wont be allocated, and all i'll recommend is turning up to check-in early to get a decent pick. Otherwise if half the flight are not having meals, half the flight is presat. If another quarter of the flight has decided to prebook their seats, thats only a quarter who will have the choice of what seats are left.