My son was in a similar situation 3+ years ago - he was advised by the Univ that he needed Further Maths AS and the school couldn't timetable this into his schedule.
In the end he studied this himself using the resources of the further Maths support programme http://www.furthermaths.org.uk/
The Further Mathematics Support Programme (FMSP) is a government-funded initiative supported by the Department for Education and managed by MEI. The FMSP supports schools and colleges at Key Stage 4 and post-16 level to increase teacher expertise and student participation in AS/A level Mathematics and Further Mathematics
Lots of free materials and resources, including online lectures and also specialist revision day. There are lost of free resources, bit if you speak to your school/college they will be able to help you to get free access to the subscription material (online lectures etc)
Worked very well