A lot of uncertainty yet about if we need to prove negative tests or vaccination status at the Etihad. However, I'd get your QR code sorted on the app mate, just in case they do ask. This is if you are double vaccinated.
There will be a few who don't know how so hope this helps:
Download the NHS App on your phone. It's not the Covid-19 one that detects contact tracing etc. It is literally called "NHS App". You may have to fill out some personal info on there for it to verify your identity, and then set up an email and password (use the same email address you have on record at your GP, if you have one set up there).
Once you've done that and logged in, on the homepage of the app there is an option that says "get your NHS Covid Pass". Click on that and then "Continue" and select the "Domestic" option.
This will open a page with a QR code, like a square shaped barcode and your name. Screenshot this page and it will save in your photos on your phone. That photo will get you into the stadium as proof of vaccination, if a staff member asks you.
There is also an option below that QR code to download the PDF version of it. Again, does the same thing if you want to screenshot that one.
This QR code is valid for a month from when you open it; then you'd have to do it again in a month's time. Screenshotting it to your phone will just save the hassle of needing decent internet/4g signal at the stadium. As you'll need that to load the NHS App, whereas a photo album on your phone does not require internet.