Although a great deal has happened in the UK over the past 40 years, there is still a lot of ignorance of the current situation. Some people think it has already gone too far, others do not seem to be aware of what has actually happened. One school of thought is that SI (metric) is OK for some fields, e.g. scientific and engineering endeavours (indeed, some people think SI was designed only for such purposes, rather than general use), but it is not suitable for others. For example, as Britain is an island, and the design of our road signs does not affect how we do trade with the rest of the world, (so it is argued) it is not necessary for road signs to convert to metric.
However, SI is a coherent system of units, and including non-metric units undermines the system. We buy our petrol by the litre, so it is easier to calculate consumption using kilometres; coaches and trucks have odometers recording kilometres, and speedometers showing km/h (not kph, please!) in prominent form. The argument also ignores the fact that the UK shares a border with Ireland, which completed the conversion of both its distance and speed limit signs in 2005.
The logo above was was adopted in the UK in the 1960s to indicate that a company produced SI products. It can still be seen on some British Ordnance Survey maps.
The Metrication Board was set up by the government in 1969 after the Confederation of British Industry and the British Standards Institution announced that industry was in favour of metrication. The Board's remit was restricted to educating the public and business, and encouraging the adoption of SI. A target date of 1975 was set, by which time it was anticipated that the UK would be substantially metric. More than a third of a century after that target, we are still not there!
By 1975, a lot had been achieved, particularly in industry, but also in a lot of everyday products. Unfortunately, the arrival of Margaret Thatcher's government saw the end of any further persuasion on the part of the Government. The Metrication Board was killed off.
However, it had already been agreed to harmonise our units of measurement by requiring SI, and transition dates were set for phasing out most non-metric units. UK legislation was then amended to enact these changes. 1995 saw the removal of the pound (weight) and pint (volume) for labelling pre-packed goods.
Perhaps the most significant change took place at the end of 1999; as of 1 January 2000 it has no longer been legal to sell loose products (vegetables, fruit, cheese, meat, nails, ground coffee, etc.) by reference to the ounce, pound, pint or gallon (with the exception of draught beer and cider, milk sold in returnable containers and precious metals). It is interesting to note that those who complain about possible prison sentences for traders who do not use metric do not complain that the same penalties currently exist for any publican who dares to sell beer in litres - one particularly ludicrous anomaly (especially given that nearly all pint bottles of beer on sale in supermarkets have been changed to 500 ml bottles). In fact, this happened a few years ago, when a publican was fined for selling beer in metric sizes; one wonders how much support that publican received from those who proclaim themselves defenders of freedom to use any chosen units of measurement? Pints of beer are usually spoken about in the same breath as the pint of milk in returnable containers, but there is one significant difference, in that it is now perfectly legal to sell milk in metric sizes.
Russ Rowlett's article on English customary weights and measures offers a very useful history of pre-metric units in Britain, while information specific to the UK is contained in Historical perspectives by the last Director of the Metrication Board.