There is no GCSE in Further Maths.
AQA offer a Level 2 certificate/IGCSE in Further Maths which is a challenging and worth while qualification but it doesn't count for state school league tables so many schools may stop doing it. This is the one with an A^
OCR offer Free Standing Maths Qualifications (FSMQ), often called Additional Maths and fairly often taken at Level 3 in Year 11 although the AQA one above has been overtaking it in popularity over the past few years.