Within 48 hours of the match, it says.
Going from experience, you carry out the test at home, submit your result to the NHS website, and you get a text and email saying that you have been tested negative.
The problem is that it relies on honesty - as you could easily just click "negative" rather than "positive" and go to the game anyway... and it would be impossible to disprove.
Even if they then asked for photos, it still would be very easy to falsify, by simply not putting the swab in your mouth for the test.
This is 100% relying on the honesty of the public. Which would be great if so much of the public wasn't selfish ****wits who could well ruin it for everyone.
As with most things the majority of people will do the right thing.