(I'm taking II and III this year, did I last year.)
I assume you're not doing Further Maths? That, (inclusive)or your college choice could be why you need it.
There are probably more doable questions with only C1/C2 than you expect. For example, looking at the Pure questions on 2004/I, you can probably do (no promises mind, but I don't think there's any C3/4 in them) 1, 2 (have you done a.p.'s and g.p.'s?), 5, and maybe 6. It's as much experience often as specifically knowing more techniques. There are occasional questions on STEP III that don't require anything past GCSE.
Not much point starting too early though, just demoralising, and you only have 20 (21 for you) years of past papers to work with, which isn't as many as it sounds.
At this point, I think the best thing would be for you to see if you can do some, or all of further maths. Even an AS is better than nothing (FP1 isn't difficult anyway), and you should make an effort to do at least up to M2. Google the further maths network if your school won't do it.