When an airgun is rated as an FAC airgun it becomes a Section 1 firearm and will be rated as one forever.

The fact that it is now S1 rated proves the fact that it is cabable of producing more than 12 ft/lb.

I strongly advise that you get the gun tested and make every effort to keep within the law with respect to this. If it is indeed FAC rated, then the seller and the current owner are committing serious offences......