ID check for buying calpol

Today I was trying to buy a bottle of Calpol for my baby boy in a Sainsbury’s, but was asked to show my ID. I am a 29 yo Asian female, who does not looks like under 18 at all. I asked if I can use a photo of ID to show my age, and sadly they said no. It was not the first time that happened. Several months ago, when I try to buy paracetamol from the same Sainsbury’s, it happened. And I have to go back home and bring it, then they don’t bother to check that. What bother me the most is that, they never ask me when my husband is with me. I am tired of being told that I cannot approve that I am 18, and I don’t want to bring my ID with me all the time since it’s easy to lose. Does anyone know what’s the criteria of checking ID when someone buying medicine? Or does anyone have similar experience?