Im A/G at Wellesbourne at the moment, and just recently passed the FISO exams. To be A'G all you need is an RT licence, and approval from the Airfield manager.
To Be an AFISO, you need to take 2 exams at Gatwick,
1- FISO laws and Procedures
2- Nav and Met (your exempt if you hold a PPL)
Then the licence will be issued, but you need validation in the aerodromes tower that you will work in, and you need to have completed a minimum of 40 hours in the tower under supervision, before the validation can take place, and once this has been completed you cn be a validated FISO.
Hope this helps,
Andrew