Ash - I have records of approximately 50 / 60 original UK pre-K1 CB750's (pre-internet days, a couple of us created a database of UK survivors, based purely on observation, ie, seeing them out there in use!)
Steve, the above records contain details of 4 genuine original UK sold K0 CB750's, which certainly did exist back then in the '80's / '90's.
Knowing the original UK registration numbers (license plate numbers to you), it would be possible to check UK vehicle computer records on-line and ascertain whether or not they still exist. Not a foolproof system, as the authorities are not always notified when a vehicle is destroyed, and it is possible for an owner to change the registration number.
Chris R.