Nothing and nobody has stopped UK citizens leaving Thailand to go to the UK. There have been direct flights available at least on a weekly basis, every single week (or more frequently) during the past year.
EVA, as an example, were charging a little over 400 GBP for a one way direct flight to London from Bangkok.
Your friend could have taken any of these flights, but instead chose not to.
It is only in the last few days that everyone travelling to the UK is required to show a negative Covid 19 test in order to board their flight to the UK.
The Br