Really sorry to hear this tragedy mate. Surely the Police and DVLA should be investigating this situation not just confescating your car. Did you have the responsibility of putting it through the SVA test etc etc when it was imported to this country?? If not did you take ownership (paperwork wise) of the car after 1st registration or has this problem arisen as a result of you trying to register the car with DVLA??
The Police should be tracking down how the car came into the country i.e. the importer and if they find that the importer has anything to do with the car being stolen (or knows that it was stolen) in Japan then you should be able to take this importer to court and win.
It will no doubt be a lengthy ordeal but if you get your money back and are able to either replace the car or pay off the loan then it will be worth while.
I would be contacting the Police, DVLA and your Solicitor for advice on how to proceed. £150 an hour is money well spent imo if you are able to get a steer from someone with a legal background.
Goodluck in this mate, real sad thing to hear.
Ant.