When the ABV drops below 1.2%, I think.
This definition of beer is from the 1992 Finance Act, but the formula seems to be in use in lots of legislation:
[quote:8c9nkecm]any liquor which is made or sold as a description of beer or a substitute for beer, and which on analysis of a sample thereof at any time is found to contain more than 1.2% vol.[/quote:8c9nkecm]