I'm in the process of doing the opposite. (ATPL/H to A).
Only a couple of schools have been approved to do the course. As per normal the CAA were no use, when contacted.
Your best bet is to give Bristol ground school, or GTS in Bournemouth a bell. They can both get approval to do the course.
At present I'm having to do 4 exams. P of F(A), Perf, Ops Proc, and a composite bridging paper.
My guess is you'd only have to do P of F(H) and a Composite bridging paper.