Not instantly recognising the importance of SSJ. Recalling the early years of Jaguar there was an SS model, but where the J would come into that I am not aware. The problem was that the Jaguar SS was eventually associated with the !Removed! German war regime and Jaguar had to drop the SS name because of that very matter. As it happens the SS model was a gorgeous looking car and in todays market are worth a fortune.
All I could suggest is that you contacted one of the many dealers in registration numbers and seek a valuation from them. A lot of any value could well be, or not be, in the remainder of the Registration Number which, for understandable reasons, you have not quoted.
Good luck with your quest and Best Wishes, Regards, John.