Are those partner airlines prepared to give back the money that TUI has/will pay ? If not, is TUI prepared to give a 100% refund to the person who booked the holiday, while still being contractually bound to pay the airfare to the partner airline ?
Austria and Italy have both pretty much banned the entry of tourists coming from the UK for the time being.
If I had a flight booked from London to Verona with Ryanair... I would expect Ryanair to still run the flight and keep the money on the basis that a seat was available to me if I had turned up at the airport... the fact that I would have to spend 5 days in quarantine and ruin a holiday would be something Ryanair would tell me to take up with the Italian Govt