The selling of certain goods must take place in fixed sizes called 'specified quantities'. But, the specified quantities regulations for selling alcohol differ depending on whether it's sold by the glass or the bottle.
Specified Quantities Alcohol (by the glass)
Still wine: The rules apply to measures of 125 ml, 175 ml, multiples of 125 ml and 175 ml
Port, sherry, or other fortified wine: Measures of 50 ml, 70 ml, multiples of 50 ml or 70 ml
Gin, rum, vodka, and whisky: Measures of either 25 ml and multiples of 25 ml, or 35 ml and multiples of 35 ml (not both on the same premises)
Draught beer and cider: Measures of a third, half, two-thirds of a pint and multiples of half a pint
Note: Specified quantities do not exist for sparkling wine or for yellow wine sold by the glass. The same applies to most spirits excluding rum, gin, vodka and whisky.
Alcohol Packaged in Bottles, Boxes (or similar)
Still wine: 100, 187, 250, 375, 500, 750, 1000, 1500 volume by millilitre (ml)