Thanks for the clarification, I'd only read the linked article and not the website (which to be fair to me, wasn't mentioned in the article).
The challenges I presented are still valid. Sure, they'll "ensure accessibility for those with...", but it never works out that way. The closest thing I can think of is a driver's license being our de facto standard ID card in Ontario. My brother, who doesn't drive, doesn't have one, so he has an "age of majority card" - and nobody knows what the hell it is. I think something like this proposal would have the same effect. Sure, it's technically usable by those without a smartphone, but in reality, not having one is still a massive challenge.
A simple read-only (write-once, technically) smart card that keeps a picture of you and can be protected by a PIN or password is a much better idea. It doesn't need to be updated when new information is stored. It's cheap. It's accessible to everyone. It's superior in every respect. It can replace driver's licenses (a challenge here in Canada, and the US, due to differing areas of federal and provincial/state jurisdiction). A smartphone app is an implementation, maintenance, legal, and security nightmare just waiting to happen. We all know it's gonna cost a fuckload more than £10M to keep it running. Just create a smart card that's basically a database key; there's no reason the data itself needs to be on the card beyond a few simple points, maybe picture and date of birth so it can be used to verify age of majority.
None of this even mentions my objections to making it mandatory to carry ID. Not in my Canada, thank you. I personally see requiring me to carry ID to walk my dog, or forcing me to identify myself to any government official any time other than when I'm requesting a government service or I'm being arrested as an egregious offense to my right to be left the hell alone.