Don't forget that TUI UK is part of the wider TUI group. It all depends how demand is across Europe as a whole, meaning the fleet can be switched around. Germans for example could be more inclined to book a holiday than the British if travel advice is different to what the UK has put out. No-one really knows which way it's going to go.
At the bigger bases where there's higher frequency routes, the 757's and 767's could be used on routes that may have been twice daily or daily, and consolidated due to lack of demand. It could also be a time to get aircraft in for maintenance that may have been done through the winter.
Until everything falls in to place and a better picture of what will happen is seen, it's difficult for anyone to judge. We could see a big spike in Winter holidays this coming winter if people feel the need to get away and this summer is a write off for some. Also remember it's not necessarily TUI Airways that make the call. It's TUI holidays. There may be consolidation on what hotels are being used, meaning less room capacity is needed to save money, whilst not ideal flying half empty planes, it could mean the reduced hotel capacity is filled.