My sister was at a private school in the UK and they were given drinks(a single glass of champagne) at certain events even before they turned 18, with permission from parents. Like you said, it's only illegal to buy alcohol under 18, but there's no problem with consumption itself.
I was given Buck's Fizz (Champagne and orange juice) when I was 5, at the birthday party of a super-posh boy who lived in the village. His grandfather didn't even ask my mum first. Upper class people apparently have different rules.
At Imperial, when the Google London office was new, the feedback from staff for why so few students hung around at their recruitment event was the lack of alcohol. Overturning this HQ-imposed very American policy apparently took significant effort.
