I don't agree with his policies or his party ideology either, but I think it is incorrect to call him an idiot. He knows very well what he is doing, and I think he is leading the country competently, in the sense that he is not blundering around stupidly and making the overall situation significantly worse for the whole nation.
Cameron has been a competent prime minister. He hasn't screwed up in any major way, the economy has significantly recovered, he's survived the Scottish referendum and the coalition has been successful in moderating his party's reforms. But his party and Clegg's are doomed in the next election.
We keep using imperial in our daily lives because they are easy when you don't need great precision or a deep understanding of the science behind; 1 pound is about the weight of a large handful of something - easy to relate to and precise enough for many purposes.
As generations are born and die, the old system falls out of use and people use the new system instead. I don't see that as a reason to keep on using it when it was gotten rid of for a reason. If I want to approximate something I can always say a litre, or half a kilo, or 50 meters or whatever.