Since the introduction of the Licensing Act 2003, there has been a lot of confusion over drinks with meals as the Law concerning ages changed.
The Law as it stands is...
The Licensing Act 2003 makes it an offence: to sell alcohol to an individual aged under 18; to purchase alcohol on behalf of an individual aged under 18; and for an individual aged under 18 to consume alcohol on relevant premises.
The only exception is that 16 and 17 year olds may drink beer, wine or cider with a table meal in relevant premises, where accompanied by an adult aged 18 years or over.
Bar snacks will not count as a table meal.
So unfortunately Bexy, your Mum is breaking the Law by buying you the drink, and you are breaking the Law by drinking it.