Registering a new vehicle without Reg.number

had to register a 1971 jag import on the old system for test as he needed to mot it in order to register for uk use,we are allowed to test a car with no plates but it had no visible vin(and at that age doesnt need one for an english test) but no plates were fitted,had all supporting documents from diplomatic/italy quoting engine/axle nos etc but system wouldnt accept it without a reg.entered reg as unk nown in order to test with engine/axle nos etc entered on advise along with the fact that the system needed updating to cope with crap like this which was legal for us to test but that the computer couldnt cope with.guess it didnt happen......