TUI are probably desperately trying to hold onto cashflow for as long as they possibly can - lack of cash can easily make a firm go bankrupt.
About a week before your holiday is due to depart you will get a cancellation email if it's not possible to go ahead, trying to persuade you to accept a voucher or suggesting an alternative holiday (ie Spain), but permitting a refund if you insist on one. If the Greek Govt are refusing UK flights, I would not have any expectation of Greek holidays happening in June - start making plans for a trip to Devon instead
Until then, you just have to wait