How to get better at theory Watch
Anything would be greatly appreciated.
Also musictheory.net! I found it really useful.
Also work your way through graded theory workbooks. When I did my grade 5 I did abrsm, and the workbook for that was pretty good. I'm doing grade 7 now though and prefer the trinity ones, as they give a bit of an explanation for new topics. Grade 5 theory is a pretty useful thing to have, especially if you want to do a-level music/music at uni.