I am on a fly cruise with P&O this Thursday 1st September and have read the rules. You do not need evidence of a test now although you have to verbally confirm you have tested negative.
The rules below that are the current rules. No test certificate required but you are asked to confirm you have taken the test (you can even use old NHS test kits if you still have them.)
Unless otherwise communicated, all guests aged five years and over must take a lateral flow/antigen test within two days prior to embarkation of the ship. This can be at any time in the two days before or on the day of embarkation. Guests will no longer need to provide a test certificate, however, they will be asked to confirm the test has been taken, with a negative result, as part of the pre-boarding health declaration at the cruise terminal/airport. Guests under five years of age are exempt from pre-travel testing
Adequate tests include over the counter tests, providing they meet the criteria below. NHS tests are now only issued for those in medical need. However, if guests have any remaining NHS tests, these would be accepted.
Accepted antigen tests include:
Rapid antigen test
Viral antigen test
Antigen Lateral Flow Fluorescence