Unsure about uni course/reapplication

I applied to uni last September and have a deferred (2016) unconditional offer from Edinburgh to study chem eng and I got A*A*A* in maths, chemistry and physics, an A in AS further maths (which I sat this year) and a B in AS english lit (pretty irrelevant).
Over the course of the last year I have been wondering a bit if chem eng was the right choice for me but put it mainly down to the kind of nerves everyone gets surrounding a big decision like this. I'm on a gap year at the moment, doing a year in industry with a materials research place. I'm now wondering if either chemical physics (offered at Edinburgh) or materials science (not offered at Edinburgh) would be a better choice for me. Obviously the former is an easier change, but I'm unsure as to what to do. I don't feel like I'm in a strong position to write a personal statement (I don't have much materials specific stuff to talk about other than journal articles and a bit from my placement which only started this week) and I certainly don't feel as confident writing a personal statement as I did this time last year.
If I withdrew from my offer and then subsequently reapplied to Edinburgh for chemical physics, would they be less likely to make me an offer (as I rejected their previous one)?
I could also reapply to Cambridge if I withdrew, which has been an idea, but I'm not sure if I want to go through the process again (pooled but not fished for chem eng, although I think my math has improved significantly and that's what let me down). The course really appeals to me but I don't know if not being in a sixth form environment would disadvantage me in the interview (as I may have forgotten some stuff).