Thanks for the reply guys.
Regarding the allocation of 787 aircraft, the staff at Thomson shed some light on this. They told me (although I'm sure their management won't thank them for this), that although certain flights are sold on booking as being 787 Dreamliner flights, and indeed will be, that some flights may also be using the 787 depending on demand and aircraft availability. The impression that I get is that they lure people onto the more expensive flights, selling them as dreamliners, but other flights may get a dreamliner if the demand is there for their increased capacity, and they're not booked up on a long haul route at the time.
In any event, this isn't about what aircraft I'll be on per se, I'm not too fussed about that as I love the older generation Boeings after many happy holidays having flown on them, it's more my own curiosity as to how they map the seating when they use their long haul aircraft for high capacity short haul routes. All I know at this point is that it won't be a 737, as Thomson do not offer 'Seats with extra legroom' on their 737s, so the fact I was able to book this extra at booking suggests it'll either be 757, 67, or 87.
As a side note about the use of exit seats as seats with extra room, I believe Thomson actually sell these differently as 'Seats with extra SPACE', not 'Seats with extra LEG ROOM', the point here being that children cannot be allocated to exit seats, where as they can to seats with extra leg room that are spaced further for comfort.
I did wonder whether in the case of 767 and 787 on short haul, the premium economy seats are used as the extra leg room seats, but wasn't sure if this would be the case, or whether premium economy is removed completely for more seats.
Many thanks