a few things
a few things:
1) as others have pointed out, US and UK units are for the most part incompatible. (Why? Well, that's mostly because the US is stuck with 1770s units.. the UK redefined many of the units during the 1800s, but by that point they were in no position to dictate any changes to US units) .. a few units are interchangeable (an inch is exactly 2.54 cm on either side of the Atlantic, and I believe a degree Fahrenheit is the same in either country as well) but many are not. The units of liquid measurement (ounces, pints, gallons, etc.) are the most common point of confusion. (The US still uses the old base 2 system [so 1 pint == 2 cups == 16 fl oz), but the ounces are larger.)
2) I'd guess that this EU decision is based more on easing trade with the US than with helping anti-metric crowd in the UK. US law (specifically the Fair Packaging and Labeling Act) requires dual-labeling in the US.. most products must be marked with both US and SI units.
So a gallon of milk sold in the US is labeled "128 fl. oz (3.79 L)" and a 2 L bottle of Coke is marked "2 liter 67.6 fl oz (2 qt 3.6 oz)" .. (why does it list both ounces as well as mixed quart/ounce? and why does the second [incorrectly] say "oz" instead of "fl oz"? I don't know, but I'm looking at a Coke bottle as I type this.)
Yes, we buy our milk in gallons but we buy our Coca Cola in liters. Silly, isn't it? Note however that our aluminum cans [not aluminium, but that's a different story] are still 12 ounces (and are thus marked "12 fl oz (355 mL)" under current law) and one can sometimes find 16 fl oz or 20 fl oz plastic bottles alongside the 500 mL or 1 L bottles. I've seen all four sizes on the same shelf at the store ... very silly.
3) Manufacturers in the US have been trying to get changes made to the FPLA to allow for metric-only labeling in the US. With this change in EU law, I guess that puts less pressure on the US to change its law. (But I imagine companies will still press for the US change.. many companies would like to drop dual labeling.)
4) Schools in the US teach both US and SI units. Students are expected to use SI units in science classes. Most consumers are at least somewhat aware of SI units. Not just in terms of 2 L bottles of soda.. most will have encountered the Nutrition Facts labels on products (which use SI units, except that they still use the deprecated kilocalorie instead of the preferred kilojoules) or will have taken medications with dosage information in SI units. Most US consumers don't know how to convert between units (even the simple ones, such as a liter being almost the same as a quart) and will use US units for most daily activities (measuring temperatures, judging distances, cooking, etc.)
5) NASA tends to use SI units. See, for example, their recent press release which promises to use only SI units in the upcoming lunar program. The problem with the Mars Climate Orbiter was due to the outside contractor Lockheed Martin. Whereas NASA had asked for some information in Newton-seconds (an SI unit), Lockheed Martin supplied the data in terms of foot-pounds (a US unit). NASA should have double-checked the units, but the fault lies with Lockheed Martin for using the wrong units.
The wikipedia article on metrification in the US is both informative and entertaining: http://en.wikipedia.org/wiki/Metrication_in_the_United_States
Biting the hand that feeds IT
