Usually the testing is based on the day the test is done and time has no factor. So if you tested on Thursday morning you would/could be fine until Saturday at midnight. That could have changed, but the only tests historically I've heard timing was an issue was the PCR tests. Pretty much everything now is antigen and works on the calendar day. Another option would be if you have access to a home test do it for yourself the day you fly. The test kits we have here in Canada are the same as the proctored pharmacist used for our official test. You have the comfort of knowing you are negative, mask up on the plane, and have the test done in England. We flew out of Gatwick (on way home) and there was a testing clinic right in the airport. Cost 35 pounds per test and had result in about 20 minutes. Then all you have to do is take train, car, bus, whatever to Southampton terminal. Not sure if they had testing right at the dock, but if they do suspect it may be more $'s. Best of luck.