The real question is I have a UK originating iPhone that i use just for photographing. I do not live in UK, but the phone is not registered to use gsm in my country since it costs 3 times the price of said iPhone to do so.

If I update, will it show the ID prompt to me? Because I sure as hell do not have UK ID, so the phone will literally become a brick.

If you're very quick about it, iOS 26.3.1 is still signed so you could do the update and downgrade using iTunes if it does ask. You would have to factory reset the phone to downgrade again though and Apple could potentially stop signing it at any point leaving you stuck on 26.4

Why not just ban smartphones for people under 18?

Because then they don't get that sweet, sweet personal info from ya

Seems like if you’re already using Apple Pay you’re fine. It’s still an egregious overreach and pointless but at least it’s not demanding a full face scan.

Because that's even worse than just adding age verification.

That's what these evil people want

Just get a Pixel and install GrapheneOS on it.

Some of my friends don't understand why this is an overreach, "i am over 18 what do i care"...

I'd imagine it's location based and not tied to the hardware. Otherwise you could just import an iphone from outside the UK to get around the age check.