I'm planning in self-studying for an international Further Maths A-level at Edexcel and my second option is at CIE cambridge or something like that.
Anyway, I've already studied S1 and I'm going to do D1 in school. Edexcel requires six units, so I'm thinking of doing, S1, S2, FP1, FP3, D1, M1 - this still means I've gotta self-study S2, FP1, FP3, D1, M1 since I have not done D1. But I figure S2 will follow on nicely from S1 and it is also assumed knowledge for the CIE board.
For CIE, you need to know M1, M2, S1, S2, FP1, FP3.
If for some reason something fails at Edexcel, at least I know that I can try CIE because for edexcel you can do either FP3 or FP2, so if I study FP3, I can still try the other exam board.
Anyway, some people think FP3 is just as hard as FP2, but I would go for FP3 because of the reason above. I'm doing M1 over S3/FP2 purely because it is needed for CIE and I imagine it would be easier than S3/FP2.