I'm currently doing Edexcel maths/further maths and I've done C1, C2, S1, M1, FP1 and got D1 next week. D1 is probably the easiest to self-teach, it's not as hard as the others. It's not even maths. Even if you're doing OCR there isn't much of a difference, apart from S1.
ukmathsteacher is also someone you may wanna watch on youtube, he has youtube videos on all of D1 and FP1 (maybe other modules as well). Plenty of other youtube maths videos are well which are helpful.