Well I would have been fuming. It is not your decision to make whether this child should or shouldn't drink alcohol at 15. It is her parents decision she is still underage and a minor. You should have checked with her parents and if that was not possible not have given her any.
This is quite an interesting comment. In 1989 the concept of "parental responsibility" was introduced, to replace the previous system of parental rights.
This does not mean that parental rights were abolished, but rather that the emphasis changed to one of a parent's duties towards their child rather than their rights over them.
The incentive for this change, as I understand it, was the decision in the case of Victoria Gillick vs the West Norfolk and Wisbech Area Health Authority which also led to the concept of Gillick Competence, mentioned by other PPs. Although Gillick Competence refers specifically to medical decisions, the change to parental responsibility seems to have been intended to widen the scope of that decision, as in this statement by Lord Scarman:
The ruling holds particularly significant implications for the legal rights of minor children in England in that it is broader in scope than merely medical consent. It lays down that the authority of parents to make decisions for their minor children is not absolute, but diminishes with the child's evolving maturity. The result of Gillick is that in England today, except in situations that are regulated otherwise by law, the legal right to make a decision on any particular matter concerning the child shifts from the parent to the child when the child reaches sufficient maturity to be capable of making up his or her own mind on the matter requiring decision.
It is clear to me that the intention was to allow for the gradual transfer of the right to make decisions regarding a child (child in the legal sense) from the parent to the child. Bearing in mind that a young person can leave home and live independently from age sixteen, it would be reasonable to suppose that all parental rights should be transferred by this point, leaving parents with just the duties to provide for their child for the next two years - until they reach eighteen. In Scotland this is specifically stated - a parent may only advise their 16-year-old, not direct their behaviour. Justification for this could be that many of these rights are independently granted in law. For example, apart from the right to live independently, a 16-year-old has the right to obtain their own adult passport (without requiring parental consent), work full-time (provided they have reached the school-leaving age - which is actually not an age but a fixed date), make their own arrangements for KS5 education, etc. With these rights comes the liability to taxation.
Since the transfer of rights is supposed to be a gradual process, it would also be reasonable to expect most rights to have been transferred by age fifteen, and bearing in mind that drinking alcohol in a private house is not an illegal activity, then perhaps the decision to drink alcohol or not should be made by the girl, and not by her parents.