Gin, whisky, rum, and vodka are the spirits which must be sold in measures of 25ml or 35 ml, unless served in a cocktail of three or more different spirits. Which is served is solely up to the publican.
Any wine or other spirit can be sold 'by the glass'. If the size of the 'glass' is not defined on the displayed price list, the publican can make the quantity any amount he likes. But if the measure is displayed, no variation on the amount served is permitted.
Drinks lower in alcohol, such as vermouth, (and Baileys), are normally served at 50ml. If the pub provides less, they're being mean. Complain like hell, and never darken their door again.