Although technically a country committed to the metric system the UK actually operates a hybrid imperial/metric system. Liquids such as milk, as well as beer, are still sold in pints (though the law requires them to be labelled in litres as well as in imperial units, resulting in the oddity that the most commonly available containers in supermarkets are 1.136 and 2.272 litre plastic bottles (= 2 or 4 pints).) In addition, long distances are still measured in miles, and cars have speedometers that read in miles per hour (with a smaller secondary scale that reads in km/h.)

The existence of this system, and the anomalies within in, can be ascribed to a combination three factors: familiarity, ease and safety of use, and price awareness. Awareness of distance and speed in terms of miles and miles per hour was quite ingrained at the time of decimalisation, and conversion of speed limits into km/h could have been problematic, since it would be likely to lead to confusion as to what was the legal maximum speed in a given place; at the very least, the distraction likely as drivers performed conversions in their head was likely to be dangerous. Similarly, placing distances on road signs in two different scales - miles and km - would have meant drivers spending longer looking at signs and less time looking at the road - another safety issue. Because road signs were never converted to show distances in km, several successive generations of Britons have grown up thinking in terms of distances measured in miles.

With the exception of athletic events (where international standards have to be accepted), Britons tend also to think in terms of medium distances in terms of yards, but very short measures are much more usually given in millimetres and centimetres than in inches. This is an ease of use issue; inches are divided into eighths and sixteenths and are significantly harder to work in, measure and calculate than their metric equivalents. The adoption of a decimal coinage was eased by very similar practical considerations - the old British currency had pounds divided into 20 shillings of 12 pennies each, making many straightforward calculations quite difficult.

With regard to pints, the main issues were existing packaging and concern about getting short measures charged at the same price. One pint is 0.56 litres. If a metric system had been rigorously imposed, it was thought the most likely outcome was that people who had once drunk pints would be getting served half-litres - less alcohol - and there was significant concern that brewers would take advantage by charging the same price. In addition, metrication happened at a time when milk was still commonly distributed by milkmen, using tens of millions of glass pint bottles, and beer in pubs using pint and half pint glasses. Lobbies for both industries successfully protested the cost of making a sudden change to dispensing in bottles and glasses made for metric measures. The fact that plastic bottles of milk are still sold in pints even though the types of plastic bottles used were introduced well after metrication must, I think, be ascribed to the issue of familiarity.

A very similar issue has played out in a different way with regard to fuel prices, which are given in litres rather than the old gallon. The main reason for this change is that the cost per unit, which has to be prominently displayed by law at each garage, looks significantly more affordable when displayed in litres than in gallons. For example, fuel sold at £1 / litre looks cheaper than fuel sold at £4.54 / gallon, particularly to drivers old enough to remember the £1 gallon.

The whole issue is still the subject of ongoing campaigning and changes to legislation were debated as recently as 2009, not to mention forming a significant part of the subtext for the Brexit debate, but discussion of this falls within the bounds proscribed by AskHistorians' 20-year rule.

Sources

RG Liptai, Metrication: Managing the Industrial Transition

Dominic Sandbrook, The Way We Were: Britain, 1970-1974