All licensed premises must dispense Gin, Whisky, Rum and Vodka in volumes of 25ml or 35ml (or multiples thereof ) using a stamped measure and there should be a notice on the premises declaring the chosen volume (you can't mix the volumes).
The industry standard is to sell all other spirits in the chosen volume but it is not a legal requirement. If the quantity of the product sold is declared on the price list then a stamped measure must be used.
If you mix any three spirits or more they do not need to be measured.
Port and Vermouth is usually served in a 50ml measure but this is not a legal requirement.