If it's LHD then that is fairly easy, I don't know how to procure a car from Japan but basically once it's in the country you need to get an MOT and SVA test, then hand the certificates to the DVLA, pay some money and you are done. For SVA you can either sent it to a specialist to have all the changes done (mainly lighting), or buy the test manual and check it cover yourself. Much of the manual is irrelevant for FMVSS (US safety standard) approved vehicles and LHD is exempted for quite a lot of things, eg. folding mirrors.

RHD is trickier than LHD, basically for a RHD non-EU spec vehicle less than 10 years old you need to do an ESVA, and usually than means "renting" an ESVA certificate from somebody who has had an ESVA test, because the testing is thorough and expensive. the difference is that the gov't wanted to restrict the number of vehicles imported from Japan because it was impacting the UK sales. The official reason was supposed to be safety concerns, but I don't think any of the most popular imports are substantially different from the EU-spec models.

edit: I forgot to mention costs - basically take the purchase price, then add the ocean shipping cost. The tax due is 10% for import duty, then VAT on top of the purchase price+duty. You can have a shipping agent deal with all this, cost is around £100. I haven't shipped a car from Japan, but when buying from the US I've arranged all the shipping myself, although from Japan it might be best to buy from a dealer who arranges all the shipping for you.