As long as the entry criteria for the destination country are satisfied the test may be performed at any accredited centre within the agreed timeframe of your scheduled time of departure and according to the regulations of that country. A valid certificate should be brought along with you to the airport prior to checking in and usually your airline will review it for compliance prior to check in. 


It is the sole responsibility of passengers to clarify in good time, with their airline, the testing requirements needed for their destination as the situation is under constant change. Information can also be found at  https://www.gov.uk/foreign-travel-advice