It is, but it can be done in just one generation. The generation that is raised metric finds it even harder to think in imperial measurements, since the imperial measuring system is significantly more complex. Source: I grew up in a fully metric country.
I just looked it up, apparently the conversion to metric happened around the time that I was born. I had no idea, I thought that it was a decade or two before that.
i.e. if the USA had tried a bit harder in 1975, they would be done now.
Also, the UK is not "fully metric". If it was, there would not be road signs showing numbers in miles and miles per hour. You can't stop people from buying beer by the 568ml, but if the UK was serious about metric, official communications and education would be all in metric.