S1 is simply a continuation of what you learnt in GCSE maths. It's obviously more advanced, but I found it enjoyable and easier than M1. I've not done D1, but from what I've heard, it's harder to score there. It would be better to check with someone who has done it before though. What are the other subjects you're doing for A levels? S1 is useful if you are doing psych or bio for example.