I just took gcse last week but my teacher gave us a huge lecture in our last lesson about the difference between harmony and tonality:
Basically, tonality is just whether something is major, minor, modal or atonal, and whether it modulates.
Harmony is consonant or dissonant.
Harmony is the actual chords used , for example, are the chords the three primary chords or are secondary chords used? Is the harmony rich with lots of added chromatic notes and full chords, or very simple, maybe just the root of each chord used?
I hope that helps but maybe someone who is taking AS should help too- as my teacher kept saying- this is very simple, it's only gcse!