Two hundred years or more ago, as countries entered the "Modern Era" so they unified, simplified and extended the reach of systems of weights and measures. Local and highly particular traditions disappeared, as did - in some cases (notably Germany) - local currencies. The metric system is the most obvious expression of this move to the Modern Era, and close behind, the decimal system
The UK managed to convert to a decimal currency but has never made it into the Metric system - though it is affected by the fact that most of its important trading partners use it. As a result, the UK is now pre-modern, with an incoherent jumble of systems in use.
Just visit any supermarket. Here you can find pints for some liquids, liters for others. Grams and kilos on one shelf, ounces and pounds on another. In Cornwall, maybe they still sell potatoes by the gallon.
Weigh yourself on the bathroom scales, and chances are you will use pounds and stones (whatever those are) rather than kilos. But your medications are normally measured in milligrams and grams.
Go to a fabric shop and you may find meters or you may find yards. Buy petrol and it's in liters, but distance measurement is in miles not kilometers. And, to rub it in, road signs show fractions of miles rather than decimal points of miles - as you approach the Channel Tunnel, you are counted down from two thirds of a mile to one third of a mile ...
Don't even think about whether your plane is being navigated in feet and miles, or meters and kilometers. Confusion on this matter has caused accidents.
My guess is that teaching in schools reflects the incoherence of the society outside. Children learn how to use bits of different systems, and none of them very well. They have no idea of how powerful a tool a unified system can be.
The UK also, of course, has a pre-modern political system - a monarchy with the usual trappings of odd local rights and privileges (ownership of swans and such like); an unelected second chamber; a first chamber designed to remind its Members of 19th century public schools.
The UK is a basket case. Expect worsening growth figures and more Imperialist adventures, with some weapons calibrated metrically and some Imperially. That's one reason why the wars will be lost.