As long as the delay to your flight does not compromise the time validity of your certificate according to that country’s requirements it should still be valid. For example, flying to Hong Kong requires a negative certificate within 72 hours of your final departure therefore a delay of a half a day will not invalidate the certificate.
Regulations and restrictions are changing rapidly so please check your airline website or destination government website for the latest up to date requirements.
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