You should really contact the BMAA to establish what you need to do if you wish to become a Microlight AFI.
However, the minimum flying experience required before you can start the Microlight AFI Course is 100 hour as PIC, of which a minimum of 60 hours must be on Microlights, including 5 hours on the variant to be used for the course.
You must also have held a Microlight PPL without operational restrictions or a NPPL (Microlight) for at least 8 months and have passed both a pre-entry flight test and written exam before starting the course.