It’s not easy finding easy-to-digest and easy-to-follow legislation online - it tends to be in huge, impenetrable blocks, and this one seems to be no exception.
However, I did find this from Bromley Trading Standards (of all places). They state it is EU Law, transposed under current UK legislation until such time as it is superceded. Unfortunately, the quotes read backwards, it’s not the easiest to copy-and-paste.
The alcoholic strength must be in the same field of vision as the name and net quantity of the food. This means that you must be able to hold the product in such a way that all three pieces of information are visible at the same time
plus or minus 0.5% for beer and wine with a strength of up to 5.5% volume
- plus or minus 1% for beer and wine with a strength of greater than 5.5% volume
- plus or minus 1.5% for beverages containing macerated fruits or plants
- plus or minus 0.3% for all other alcoholic beverages
However, the labelling of alcoholic beverages differs from the labelling of other foods in several ways, as follows:
Alcoholic beverages must be labelled with their alcoholic strength to a maximum of one decimal place in the format ‘x% vol.’ (where x is the strength of the alcohol). You can alternatively declare the strength in the format ‘alc x% vol.’ or ‘alcohol x% vol.’ - for example, ‘Alcohol 5.4% vol.’
The stated figure must be accurate, with the level of accuracy being dependent on the type of alcoholic beverage
Whilst I can find reference to it being within a given range, I can find no reference to it having to be in 0.5% increments. Indeed, the bottle that TWS sells suggests so too. I would concede that I know no more, and that I don’t have anymore compelling evidence other than this above that I have read, plus colloquial reference to remembering other bottles that are the same. But I don’t, at the moment, see any compelling evidence to the contrary either.