TUI absolutely will not fly if the FCO says "do not go". As to whether Greece will admit people from the UK by 17 July without still requiring potential quarantine is uncertain - only the Greek Govt can answer that....
Please have a careful read through of the FCO advice on Greece, particularly the sections on "Entry Requirements" and "Staying during coronavirus". Greece is still not sure they want potentially-virus-infested British tourists coming to their country
https://www.gov.uk/foreign-travel-advice/greece
If you really want to go on holiday on 17 July, I wouldn't be booking Greece quite yet