I was once with my mum in tescos and they refused to serve her. They asked for ID from both of us. I have a picture driving licence so was able to prove my age but my mum didn't. She did point out that it was her 19 year old (at the time) son who was standing next to her (me) and my mum looks older than I do. She had to leave the supermarket without any alcohol.
I went back in afterwards and bought it for her from a different checkout.
I've been ID'd a few times when I worked at wilkos by wilkinson's staff despite producing a staff discount card.
Its also mad that you have to be 18 to buy scissors but there is no restriction on power tools.
If you find somewhere is IDing you and you are obviously older than 18, it is most likely they have been caught serving underaged people. My local branch of weather spoons IDs everyone. They refused service to my dad as he had no ID on him. They also may be aware of mystery shoppers and people sent in by the police. We got this at Wilkos a few times but we always caught them out.