Well, for BJ8s there is another possible way to get the BMIHT data without buying a certificate.
Contact the BJ8 registry to see if the information is already recorded in the registry files. I have a friend in England who researches for me in the BMIHT archives. Consequently, I have records on cars from that source when I may not even know where that particular car is.
Send me your VIN by filling out the BJ8 questionnaire at the link provided on the main Austin-Healey Forum page. If I don't already have the records for the car, I can get them.