You can buy items in the shop that have age limitations to them if you are over the accepted age and have a child with you. As long as the staff have no reason to believe that you might be purchasing the item for the child or someone under the legal age.
Sometime this is seen in the extreme, but it is perfectly understandable. I work in a newsagents, and we regularly have under 18s ask for cigarettes and when they are refused, pop outsid to the car and ask their parents to get them for them.
At the end of the day, we care about our jobs and don't want to get fined/sacked/jailed. And we are well within our right to refuse people.
However, on this topic. Cinemas are really clamping down on ID-ing people when they buy tickets and then when you enter the actual cinema room.
So it would be unlikely that you would get away with it. I got ID'ed for an 18 cert film for the first time since I was 15 a couple of months ago. I was really shocked!