Before I start I will point out that I know how Airline seats cost more the fuller the A/C gets, I am booked with EZY from LGW-ACE on the 17 May there is a large group of us traveling, we have two more people that have decided to come along so I had a look for the cheapest flights ! £757.96 inc a £40 discount STN-ACE with TOM I was stunned to say the least ... no wonder people are leaving the charter Airlines and flying EZY and FR ect.
You have just hit the nail on the head with the last part of your comment. . . Thomson Airways is a charter airline. Infact it is is known as 'the worlds largest holiday airline'. This means the majority of the passengers will be travelling as part of an Inclusive Tour company (mainly in TOM's case, Thomson and First Choice holiday groups). Any 'flight only' seats sold are just spare capacity on the aircraft but for some reason TOM are wanting to discourage this (most likely hence the price) and want to cut the small 3% of 'flight only' sales even further to focus entirely on their in-house holiday group pax bookings!
I do agree with you though, that is a very high price regardless of if they are trying to discourage such bookings. Although even Ryanair are coming up around the £550 mark when including a similar baggage allowance etc, so forward bookings must be looking good or as it is a very busy period of the year, of course with the school half term being around this date, then they are probably trying to cash as much in as possible where they can.