In order to convert miles to km, such as when you see a road sign for your destination, multiply by 1.6. Same applies to speeds, of course, 30, 40, 50 and 60 miles per hour are 48, 64, 80 and 96km/h respectively (very nearly exactly - see a spedometer which gives both scales). Km to miles involves dividing by 1.6. One day this conversion will not be necessary in the UK.
Even though some things change slowly, they do change and although I don't expect to live long enough I feel certain the UK will eventually ditch the imperial system totally (those who scream "never" at this will then also be long dead). Celsius is already overtaking Farenheit (a German scale, interestingly enough) and who would have predicted that 100 or even just 50 years ago. The metric system is already in officially, except where it is politically unwise to enforce it. The process will be largely painless, don't worry - once it is cool to talk in km the writing will be on the wall.
agree with KARL - first pounds and ounces went, now it will be miles, and quite bloody right too, why should we be all pompous and british - what's wrong with a unified measurement system? And if we stopped being so rigid over driving on the left then a) we wouldn't have problems driving round europe b) we can buy the same cars as are being sold on the continent for a damn site less than in the UK