75% sounds like a pass to me!
Okay, I know you shouldn't aim for bare minimum. I remember doing my ATPLs and throughout the course, like you, seemed to average 75%ish, but managed to jump my overall average up to 89% between the mocks and the real thing.
I went through Oxford, so things may be a little different, but I suspect not. Do your best to learn the subject up as far as the mocks and then hammer the feedback in preparation for the real thing. I remember being told that most people's average "magically" leaps by about 10% using this method. So it sounds like your right on track for a good set of results.
Don't forget those fantastic exams like VFR and IFR comms, which only require 30 minutes of feedback overview on the morning of the exam to acheive a high 90s per centage which will drag your average up.
Like I said, I didn't study at Bristol, but everyone I met who did said their feedback was pretty spot on.
Good luck.