Great! I think I asked about this possibility months ago. But are they meant to be getting high density, all economy 788's? Says so on wikipedia
I believe G-TUIB & TUIH are going to be having their premium cabins removed with additional 3-3-3 economy being put in it's place for a total of 325 seats (up from 288) - they will (in theory) be restricted to the shorthaul network through the summer.
The order for 787-9s could open up some interesting possibilities for the airline, you get the capacity of an A330-300 - which is more than anything Britannia/Thomson have had before but with range and capability that surpasses the -8.