The FI is a rating, which is attached to your licence. Medical validity only pertains to your LICENCE. From what I can make of LASORS, the AFI rating is attached to your microlight PPL. The PPL needs at least an endorsement from your GP, though JAR CLass 2 and 1 would confer medical rights also.
Is it true PPL instructing on microlights can potentially pay more than on PPL(A)?