An IGCSE is available from CIE and is set to continue.
Really good support including videos, vocab lists, list of appropriate books and all past papers is available from the International Sanskrit Examination Resource. ISER
This site also links to courses (usually Saturdays) which offer IGCSE Sanskrit. St James School offers the opportunity for External Candidates to sit the IGCSE with them.
We have studied towards this syllabus for some time. If you are going to attempt it, start early - I would say allow 4 years. My older son has run out of time to take it, but I will report back here if my younger son manages it.