I've only had a quick look at the TUI website but can't find any reference to them making a charge for booking assistance etc - only that they advise you to phone their customer service line first to check the suitability of your proposed holiday/hotel etc before booking and that assistance etc must then be requested in advance.

Ryanair tried to introduce a charge for booking assistance but they were taken to court and had to back down over it because it was ruled that they were in breach of the relevant DRA law. The case was well reported at the time so I wouldn't have expected TUI to now try and do something similar. However, they are allowed to limit the number of assisted passenger bookings etc they take on eg safety grounds and with Ryanair, for example, you have to phone in advance of booking, check that they can accommodate you on the flight you are wanting to book and the last time I booked with them (some years ago now) I was given a reference code to then use when booking on-line. From memory this was timelimited ie the booking had to be made before the code expired.

So do you have a link to where it gives more detail about this charge because, as I said, I can't find any reference to it on their information for assistance bookings and disability requirements.