Some staff members in places like Morrisons and Tescos etc have got the challenge 21 thing completely wrong. I've had it when I was 20, and I wasn't allowed to buy alcohol, even though I had ID to prove I was over 18. The members of staff didn't realise that it was just if you don't look 21, you have to prove you are 18. I just asked for a manager or someone to come over to sort it out, and thankfully they did.
I do think it's a good thing though that this Challenge 21 scheme was brought in, I can see a major difference in my local town. When I was 16/17 it was rather easy to buy alcohol, I could even carry around a photoshoped & printed version of my passport, with the date slightly altered, and I could get away with it. Back then, our town used to be full of kids being arse holes, unfortunately, at the time, i was one of them. Never violent towards people, but doing petty vandalism etc. Thankfully I grew up, and now that this +21 scheme is in, it's nice and quiet, with only people out drinking for a good time in bars etc. Much more pleasant.
Now I'm 21, 22 on Tuesday, I don't mind getting my ID out anywhere, I always carry it on me, saves any hassle when walking into shops.