if you get anything other than a positive reaction from the barman
or pub management...dont argue...just report it to:
https://www.tradingstandards.uk/consumers/support-advice
The law
In the UK, draught beer must be sold by reference to quantity and normally in quantities of half a pint or multiples of a half pint. It is one of the few imperial measures that are still allowed to be used following metrication some time ago.
The normal quantity of beer asked for in pubs across the UK is a pint which is measured into a pint glass that bears a stamp to show it is an accurate measure.
Most pubs use what are termed brim measures – this means that they contain a pint when full to the top.
A few pubs may still use glasses that are called line measures – these are slightly over-sized glasses that have a line near the top to show the level of one pint.
Even fewer pubs may use what are called ‘beer measuring instruments’ that accurately dispense beer into a glass in multiples of a half pint.
One of the problems with using a brim measure for the sale of beer is that it is virtually impossible to get a full pint of liquid due to the frothy head that is formed when the beer is dispensed into a glass.
Over the years, the generally accepted norm is that a pint containing a minimum 95% liquid and 5% head is OK.
However, this is a legally complex area and if you ask for a pint, perhaps you should really expect to get a full pint of liquid.
As part of this project, a legal opinion has been obtained from experts. In an ideal world, all pubs would be required by law to use line measures, making it far easier to get a full pint of liquid.
However, no such legislation currently exists and pubs can legally use brim measures.