My son, aged 18, went to tescos. He bought some alcohol - fair enough. After the sale was completed, paid for and he had a receipt, whilst leaving the shop they took the bottle off him.
This is simply theft. It was his property at that point.
Their argument was that he was that his younger sister was about so they decided he was buying it for a minor. He wasn't, obviously. They can, if they wish, refuse to serve anyone for any reason. However, in this case they did serve him and the sale was completed.
So what they did was theft - pure and simple.
My son did not quite have the nerve to call 999 at the time, and they did give him his money back. Had it been me, I would have simply called the police. I think next time he may do so.
I think I may write to tescos and demand an explanation.