Most Imperial units are not British

One of the myths about Imperial units is that Imperial units are made in Britain. The truth is very different, although Imperial units were defined in the 1824 Weights and Measures Act which made several other traditional units obsolete, the origins of Imperial units, which came from medieval English units of measurement, are not British. 

Imperial units were mostly brought to Britain by the Romans almost 2000 years ago, other units were introduced by the Angles and Saxons from Northern Germany who brought their own measures, the Normans reintroduced Roman style measures, and others were later adopted and modified from medieval French variations of Roman units, known as avoirdupois and troy measures.

The statute mile, better known as the mile, is of Roman origin.  The word mile comes from the Latin mille passus ("thousand paces"), derived from 1000 paces of a Roman soldier, or 5000 Roman feet.  The Roman mile was approximately 1479 m.  During the reign of Elizabeth I of England, the 1584 Act of Parliament defined the mile to be 5280 feet or 1760 yards.  In 1959, the mile was defined as 1609.344 m or 1.609344 km exactly.

The yard is probably of Anglo-Saxon origin.  Certainly the word yard is derived from the Old English gyrd ("rod" or "stick").  There are many claims about the origin of the yard, one of them is that Henry I (1100-1135) decreed the lawful yard is the distance from the tip of his nose to the end of his thumb (with his arm outstretched), and also that a yard is 3 feet.  In 1959, the yard was defined as 0.9144 m exactly.

The foot is a measurement nominally based on the length of the human foot, but the length of a foot varies from person to person.  The Roman pes ("foot") was defined as 12 unciae (Roman inches), which is exactly how the Imperial foot is defined in terms of inches.  The notion of 12 inches being 1 foot was brought to Britain by the Romans.  The Roman foot was about 29.6 cm, whereas the Imperial foot became 30.48 cm.

The inch is a measurement nominally based on the width of a thumb.  Ancient civilisations had their own words for inch or thumb, and presumably had their own standard "inch" or "thumb".  The English word itself however comes from the Latin uncia ("one twelvth part"), the translation into English explains why there were twelve unciae (Roman inches) in one pes (Roman foot).

The pound is of Roman origin, and is ultimately descended from the Latin libra ("scales" or "weighing").  There was originally 12 unciae to 1 libra (uncia was also used to measure mass).  The abbreviation for pound is lb, which ultimately comes from the Latin word libra.

The ounce is also of Roman origin, and the English word ounce also comes from the Latin word uncia.  The translation also explains why there were originally 12 unciae in 1 libra.  The abbreviation for ounce is oz, which comes from the Italian word onza.

The known pounds and ounces are actually avoirdupois pounds and avoirdupois ounces.  Avoirdupois comes from Old French aveir de peis, later avoir du pois ("goods of weight"), and avoirdupois units were introduced into England in the early 1300s and first adopted by London merchants.

The basis of the avoirdupois units is that there are 16 ounces in 1 pound, and English avoirdupois was based on the French avoirdupois units, but adapted to fit other units in use in England.  However these are still ultimately based on the Roman units of measurements with minor modifications.

A troy ounce is so-called because this measure (along with the troy pound etc.) originally came from Troyes in France, along with the troy pound, etc.  Troy units retained the notion that there are 12 ounces in a pound, just like in Ancient Rome. A troy ounce is not the same as an avoirdupois ounce.

Acres, which are now no longer used in land registration, were introduced by the Anglo-Saxons and originally measured a strip of land that could be tilled by an ox in a single day.  The Romans also had their own acre too, the actus quadratus or acnua, which was approximately 1260 m² (by contrast the Imperial acre, which is ultimately based on the Anglo-Saxon acre, is 4046.85642 m²).

Furlongs are only used in horse racing in the UK, and were also originally introduced by the Anglo-Saxons.  The word furlong is derived from the Old English furhlang (furh is "furrow", and lang is "long").  This referred to the length of the furrow in one acre of a ploughed open field, specifically the distance a team of oxen could plough without rest.  The furlong was regarded as roughly equivalent to the Roman measurement stade (stadium).

The pint is precious to so many beer drinkers.  The word pint actually comes from the Old French pinte, which probably is ultimately derived from the Latin pincta ("to paint"), which specifically refers to painting the line on a glass.  And an Imperial pint (568.261485 ml) is not the same as a US pint (473.1764723 ml).

As we can see, the Imperial units of measurement are based on Ancient Roman units of measurements in general.  There is nothing British in the origin of Imperial units, the "system" was imposed by invaders.  Indeed the Imperial "system" is like a cross between the Roman and Anglo-Saxon measurement "systems", but mostly Roman in origin.  So far from being British, the majority of Imperial units originate from Ancient Rome, Italy, Germany, and France. Like Roman numerals, Roman-style Imperial units are really cumbersome to use, and Imperial units do not even form a system.

For more information, you can read this article and this article.