When are you flying back? Are you sure you still need a test? If you do I used Dr Donna at Medconsult. I did the antigen test for 900 baht at the time - think it’s now 600 baht - this test is good for UK entry.
No particular documents required to leave Thailand. Ensure to get a re entry permit if you have a valid visa/extension which you wish to remain valid. Safe travels.
personally I would book a return within the 60 days tr period - but ensure you are allowed at least 1 change of return date for free which most airlines are allowing at the moment.
for TR yes you should have an onward or return ticket- apart from COE, airline may well ask to see this when departing UK. In current times most airlines are flexible with changing return dates if you have a definite plan to return to UK in 2-3-4 months.
It depends on what visa type you are entering on. For tourist or visa exempt generally an onward ticket (doesn’t have to be return ticket) is required.