https://en.wikipedia.org/wiki/Thomas_Cook_Airlines
… and a hotel chain …
https://en.wikipedia.org/wiki/Thomas_Cook_Hotels_&_Resorts
I would not be surprised if the UK has laws in place, and/or are using ATOL funds, so that the existing Thomas Cook aircraft can be pressed into service for the return journeys.
For example, return arrangements have already been confirmed for those traveling from Croatia on the 23rd:
https://thomascook.caa.co.uk/customers/if-you-are-currently-...
The return flight details are identical to the original Thomas Cook flights (other than the flight numbers, which are similar).