Because there is a reactionary lobby in the UK, and old f*ckers refuse to move to the metric system because they're morons who hate change, instead insisting that young people brought up in the metric system use the stupid ludicrous primitive imperial system instead. We've got as far as degrees Celcius and selling food and petrol in grams and litres, but we have such oddities as the jack length in indoor bowls being given in metres but bowl distances from the jack being given in feet and inches, and petrol being sold in litres to enable us to drive along roads measured in miles at speeds measured in miles per hour. And if R*f*rm should by some dreadful mischance win the next election, expect this all to go backwards, the metric system to be unravelled, and currency to go back to LSD (which is what we may have to take to try and escape the dreadful reality).
As you can tell, I don't have any strong feelings on the subject.