Have to disagree with BEags a bit on this one.
You must have UK CAA or JAA-FCL licence including valid SEP Class or Type Rating, CFS Standardisation Check in last 12 months, meet knowledge requirements for grant of JAR-FCL CPL or have taken Mil Accred Scheme and a few other bits (LASORS Section H 1.8 refers).
Assuming you have met the above requirements then a CFS Cat B2 equates to FI(A) with restrictions iaw JAR-FCL 1.325, B1 with 200 hrs instruction equates to FI(A) without supervisory restriction and A1/A2 equates to FI(A) with no retriction.
No need for assessment with FTO although I'm sure they'd glady take some money off your hands if you were desperate.