That was what I thought but they clarified today:

“Hello, if you are using an NHS Lateral Flow Test to demonstrate your COVID Status, this will remain valid for 48 hours from the time you took the test. You will then need to re-test on site at the festival on the expiry of the initial 48 hour period. Please make sure you bring some some lateral flow tests with you.

If you leave the site during the festival on Saturday and Sunday, when you return you will be required to demonstrate your valid COVID Status. Please ensure that you upload your re-test to the NHS website.”

Posted By: malkybarkid, Jul 19, 13:24:57