tas and Syed: No such thing as an Assistant Flying Instructor any more (in fact not since 1/1/2000 have any new ones been qualified).
They are now: Flight Instructor (Restricted)
or: Flight Instructor
To get from the first to the second you have to have 100hrs instructional hours, have supervised 25 student solo sorties, and be recommended for upgrade by your CFI (and pay some money to the CAA for a new licence page).
The basic Flight Instructor Course (FIC) involves some 40 hours of flying, 100+ hours of ground training and a flight test (sorry, I don't have the exact numbers to hand).
Various schools do it. Suggest you ring around some of the bigger ones in the various 'Where to Fly' guides and see what they say.