Further Mathematics is a suitable course for many more students than just for the top A level Mathematics students.
AS level Further Mathematics is accessible to any student capable of passing A level Mathematics. The topics in FP1 are no more challenging than the content of C3 or C4. What is more any student planning to take a degree with significant mathematical content will benefit enormously from taking AS Further Mathematics. New ideas such as complex numbers and matrices which are in FP1 occur in the first year of many undergraduate STEM degree courses. Studying these at A level together with two more applied modules will help students get off to a good start with their degree studies.
In 2013 around 15% of all A level Mathematics students also took A level Further Mathematics. So it is likely that if you have students taking A level Mathematics then a small number of these would also benefit from studying Further Mathematics to at least AS level.