I'm seriously considering doing a FISO rating
R2112, be aware that unlike ATC, you will not hold a FISO rating, you will hold a
licence. The 2 papers I recall you are required to sit by the CAA are FISO Procedures aswell as the Air Nav & Met. As you hold a PPL you may be exempted from the latter but its best to check with the CAA first. Also, once you have passed your exams you will have a licence but you will then need to complete a minimum of 40 hours supervised training before you can fully validate it at your aerodrome.
Best of luck.