> The major European exception being the UK, which doesn't issue identity cards and somehow manages.

You have to bring in loads of other random documents to get a passport, for example: https://news.ycombinator.com/item?id=44408872

To reliably prove your age (e.g. in a store or a concert venue), you have to get a provisional driving license. If you are not qualified (maybe still disqualified to drive, or eyesight problems), tough luck, I'm not aware of any other government-issued photo ID in the UK. Foreigners previously had BRPs, but they are not issued since 2025, everything is electronic now.

There is also the "PASS" scheme where private providers (like banks) can issue plastic cards with photos, but I've heard they are regularly rejected by various venues as proof of age and/or identity, there is no legal requirement to accept them, just a recommendation. And I can imagine non-UK entities care about these about as much as they do about ISIC. TSA was definitely unhappy when I tried using ISIC about ten years ago for a US domestic flight.