Don't forget though TUI UK are up two 789's on last summer with G-TUIN/O being delivered a couple of months ago.
This frees up some 788's to do some short haul flying, like the 767's have done for years. This will increase capacity at bases like MAN and LGW. This obviously won't account for all of the extra lift needed. But up gauging a route from say a 738 to a 788, frees a 738 to operate somewhere else whilst also increasing capacity on the original route.
AFAIK both 767's were due to be gone from the UK by now too, but YG will return to operate alongside YH at MAN all summer.
Will there be any spare capacity from other TUI airlines? Rather than having to outsource is there any flexibility within the group to get spare a/c across. I'm thinking the extra two 767's in Scandi for example.