Can under-18s buy alcohol-free drinks in the UK?

Legally, yes. Under the Licensing Act 2003, drinks at 0.5% ABV or below are not classed as "alcohol" for licensing purposes, which means the age-18 restriction that applies to real alcohol simply does not apply to alcohol-free or de-alcoholised drinks in law.

In practice, plenty of pubs, bars and shops apply an over-18 (or Challenge 25) policy to alcohol-free drinks anyway, as a matter of internal business policy rather than legal requirement - often to avoid ambiguity at the till, or because the product sits on the same shelf or is served from the same tap as the alcoholic version. Don't be surprised if you get asked for ID even though the law does not technically require it.