As part of the metrication process, the pint in the UK is now mostly used as a measure for beer when sold by the glass (in pubs for instance) and, to a decreasing extent, milk.
History
The Imperial pint is defined in terms of the gallon, which was originally defined as the volume of eight pounds of wheat. Other versions of the gallon were defined for different commodities, and there were equally many versions of the pint.
America adopted the British wine gallon (defined in 1707 as 231 cubic inches) as its basic liquid measure, from which the US wet pint is derived, and the British corn gallon (1/8 of a standard 'Winchester' bushel of corn, or 268.8 cubic inches) as its dry measure, from which the US dry pint is derived.
In 1824 the British parliament replaced all its variant gallons with a new 'imperial' gallon of 277.42 cubic inches, from which the UK pint is derived.