Valid here means, the app tries to determine if you did any of the things the rules tell you not to do, like, wearing sunglasses, or using the photo of you with your best friend, or taking a full body photo when they want a face. Later a human will check, and also compare the photo to your last one. Maybe you dyed your hair blond... and grew a different moustache... But how did your missing eye grow back? And why is your nose now further up your face?
So the app catches a lot of dumb mistakes that you could make, before a human had to waste their time, but it isn't the only validation.
The UK doesn't have a requirement for anyone to carry ID. I've never carried ID.
Most young people will carry photo ID because it's needed to get into a night club, to buy booze and so on. There are policies like "Check 25" enforced in some places which tell staff they ought to refuse to serve people who seem under-25 unless they can prove they're legal to drink (18 in the UK) but (a) I don't drink and (b) I haven't looked young enough for many years now. For this purpose they can use a driving license if they have one, and there are several other acceptable forms of photo ID including some non-government ID.
If you're driving a motor vehicle police can require you to provide proof that you (and thus you'll need ID) are entitled to be driving this vehicle but it's not instant, so if it happened to me I'd put a note in my diary to take my driving license to the police when it's convenient.
Historically German occupied countries in the war had mandatory ID and so there's an association in British minds (largely false) between winning WWII and not needing to carry ID. Government has tried various strategies to try to change this but they haven't been successful.
Certain non-citizens have to physically attend somewhere at intervals so that government can keep track of them, a Russian colleague of mine would sometimes be late because she'd spend the morning waiting for her local police to confirm that yup, she was still here, still had a visa authorising her to work here, hadn't gone anywhere else. But since citizens do not carry ID the non-citizens don't need ID the rest of the time anyway although I expect many do carry some out of habit.
Many older British people won't own any photo ID. If you can't or don't drive, and never travel abroad you'd have no practical use for it. I suspect my grandmother (who hadn't driven since WWII) didn't have any ID more recent than a war-era government ID with no photograph.
So the app catches a lot of dumb mistakes that you could make, before a human had to waste their time, but it isn't the only validation.