The short answer is it's almost impossible to register a vehicle in your name in Japan without having residency there. It might mean applying for a working holiday visa just to be able to do it. I'd personally recommend doing the reverse run and buying something in the UK, which would be a whole lot easier. But yes getting a CPD carnet is the only way to get through all the countries. Make sure you sort your visas well in advance, as many of the east European countries time your visa down to the hour - not handy when you get a flat tyre (and in eastern Europe, expect to kill all your tyres, a second set of tyres, and possibly a rim or two).
Check out the Mongol Rally if you haven't done so already. We (Iron Lady imports, that is) sponsored an Aussie guy to do it, vids are here: http://vimeo.com/mandurphy/videos/all/sort:date