These vacationers are in safe countries with good infrastructure. They definitely aren’t stranded like Tom Hanks in Cast Away. However, the other airlines can’t magic up 150,000 seats, so it might mean that some people will take a while to get home.
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).
I suspect it's different operators re-using the same flight numbers and airport slots.
ZT is Titan (a short notice lease provider, which makes sense): https://en.wikipedia.org/wiki/Titan_Airways
U2 is easyJet: https://en.wikipedia.org/wiki/EasyJet
Titan Airways are confirmed as providing support:
> Following the collapse of Thomas Cook, Titan Airways are working in co-ordination with the Civil Aviation Authority to provide return flights back to the UK for those passengers currently overseas.
The operation will temporarily be the 5th largest airline in the UK, with a fleet of around 50 aircraft.