Doing C1, C2, S1 and FP1 gives you an AS Maths qualification and nothing else. You need two other applied units (eg M1 and D1 or S2 and D1 etc) to get a Further Maths AS qualification.
For Edexcel, Further Maths AS is slightly more difficult than AS Maths (depending on what applied modules you do as well). If you're good at / enjoy Maths, then it shouldn't be too difficult. But don't take on work that you don't need to - check Dentistry's entry requirements if you haven't already and look at what subjects they do, and whether sociology is a suitable A-level for that, or how they view Further Maths, etc.