The best way to import a motorcycle from Japan is to let someone else do the hard work. There are plenty of English speaking exporters based in Japan who can source the bike for you and arrange shipping, or if you have your eye on something specific you can speak to an importer based in England - there's quite likely to be one near to you. Once you have found someone to take care of the shipping, the complicated bit starts. HMRC needs to be informed and will need to see registration documents and other paperwork from Japan. Once that's done they'll usually send you a tax bill. When the duties have been paid a DVSA inspector needs to look at the motorcycle and approve it for the UK. Some modification might be needed to make it legal. Once that's done, it's time to get the V5C which can take another few weeks.

With the cost of shipping, import duties and other expenses it's not really economical to import a motorcycle from Japan these days. It was, back in the 1990s, but now the only real reason to import is to get hold of a rare bike that was never sold outside of Japan.