Right. I would strongly recommend that you contact Keele's admissions department and echo your concern. As it stands, based on what you've told me, dropping your A-level would mean that you are jeopardising your offer - conditional on CCC or CC D or D*.
I would take what your school is advising as a pinch of salt. Statistics are useless without context, the context for you being that you didn't sufficiently prep for your mock. You know yourself better than your college.
In any case, give Keele a call today.
I should add that, were your offer unconditional, the question as to dropping your A-level would've likely been more straightforward.