Update:
The "Verifica C19" App is already on the App store, at least for iOS. I downloaded it and did a brief test on my NHS-provided QR code. Didn't work. Then I tried another QR code, this time taken from the similar French App (discussed here
https://www.flyertalk.com/forum/33422264-post377.html ) which yielded a positive scan for NHS vaccine passports. Again, didn't work. I then got my sister-in-law's QR code (Italian citizen, vaccinated there) just to ensure that I wasn't doing something wrong and lo and behold it did work. The app should work with all EU-based codes so I'm not sure why the French app didn't work but here it is.