Mixture. I'm used to using SI units and school and such without any problems, but there are still some places where people tend to use Imperial. The obvious is example is on the roads (miles) and when judging the height/weight of another person which people still tend to do in feet/inches and stone/pounds. It's more of a generational thing though, I could easily say I was 6' 4" or 193cm tall, while older people tend struggle with centimetres since they grew up using imperial units. Ireland did it. And seriously, that's not too hard. Sweden switched sides of the road in 1963. Yes, but there was much less traffic on the roads back then; and the UK is a much more densely populated country than Ireland or Sweden. The cost to do it today here would be huge compared to what is was back then, especially if every single sign had to be replaced.