The short answer is Yes,you can arrive in UK with a foreign registered vehicle and get it re-registered.The UK Customs are very adept at sorting genuine travellers from foreign car dealers and my own experience(twice) is that they are 100% helpful.
BUT,getting into UK is the easy bit!!!The first series of questions you may wish to ask is can u export Jap car/plates on an a)temp basis assuming u will return or,b)on a permanent basis where u have to deregister in Japan and travel on export plates.
To do the latter would almost certainly be more trouble than its worth because even though u will have a Carnet(impractical
without this),most countries have wierd and massively frustrating travel requirements for vehicles with export plates(fixed convoys etc).No fun at all.
I have got around this in the past by sending a letter to the registration dept.of the base country saying that I am travelling on leave and will be back within x months and then,on rereg in UK,returning the plates by couriering them to the Carnet office who then return them to the registry office and then release the Carnet Bond.
Vital u check requirements of countries u will pass thro.Local Carnet office usually know about adjacent countries and keep talking to travellers thro webs like this one.
Complicated but well worth while at the end.Good luck!!