Because they are a weird middle ground. UK uses metric paper, metric weights and measures for _most_ things, metric temperature. Where Imperial starts to slip in is distance when driving (but not when running...), volumes of Beer and Milk (mainly cause we want more beer than 500ml) and body weight (where we use Stones, which is weird). There are probably some other things as well, but metric is the dominant system (for people under 60) for sure.