I believe there were multiple factors that led to the ESAT not going well, especially as I was ill at the time. However, I do think that the other points you raise are valid, and I'll definitely have to think about what I'll do over the gap year (if I do take one).
Having spoken to a few people, I do think my A-Levels were better suited to a straight physics degree as I'm doing maths, fm, physics ad computer science, as well as AS chemistry. If I do end up reapplying, maybe Oxford is the way to go.