You basically need to go to the NHS website and put in your result (I know, ridiculous) You will need to put your personal details in the first time and can get a dated screen shot of the confirmation - we do this for my lad at school.
As I understand it, however, proof of being double jabbed isn't legislated for yet, is still only advisory, as government still not ruled on outside mass gatherings.
There were suggestions something could be announced in September but nothing as yet.
This is very misleading, as nobody can demand proof of jabs at this stage, certainly not the Premier League, who can't even decide yet what to do about players who have refused the jab.