What is the law on underage drinking at home ?

We r having 16th party in garden for our daughter who is one of the youngest of her friends,so a lot of them already drink alcohol.We will not be supervising the party but will only be a few doors away and will pop in and out.Should I send out permission slips to parents warning them there will be alcohol (not that im providing it but they all bring it themselves apparently).My daughter is mortified and embarrassed that im even thinking of this .Can any one suggest a better solution pls ???