The Alcoholic Liquor Duties Act 1979 (as amended) states:
>“Beer” includes ale, porter, stout and any other description of beer, and any liquor which is made or sold as a description of beer or as a substitute for beer and which is of a strength exceeding 0.5 per cent<
A court would almost certainly rule that, for the purposes of the relevant legislation, shandy is sold 'as a substitute for beer'.
The Licensing Act 2003 states:
>“beer” has the same meaning as in the Alcoholic Liquor Duties Act 1979 <
Ergo, the sale or supply of shandy (as usually mixed in bars) is subject to exactly the same legislation that the sale or supply of beer is (because it would typically have an ABV of around 2%).
However the type of shandy which is sold in cans in newsagent's shops isn't covered by licensing legislation because it has an ABV of under 0.5%. (It's typically about 0.2%). In a bar such a product would probably be called 'lemonade and a dash', rather than 'shandy'.
So selling or supply shandy (other than where it's actually 'lemonade and a dash') to anyone under 18 years of age is an offence.