I think I recall reading somewhere that as part of Tui group the aircraft will be moved around different parts of the organisation between summer & winter seasons. (One of the benefits of fleet commonality with regards to seating.) So in winter some of the 787s may go to Scandinavia where there is traditionally a larger long-haul programme. Historically the 767s have done this in recent winters. I am sure they won't be sitting on the ground in the UK doing nothing (albeit that the winter flying programme will clearly have more slack in it than summer.)