I went with Oxbridge for A Level Maths. I haven't had any experiences on other courses or other colleges yet.
I think the best place to start is to specify which subjects you want to do and with which specific exam boards. Each online course will restrict to going with one particular exam board for each subject.
If the exam board isn't good for your subject, no matter how good the college is, the specific course might not be what you want.
A side note is that you might end up having to pay for everything yourself as opposed to getting everything funded through the college/6th form. You would need to check with the Student Loan Company to check though.
If you want specific advice on my experience in A Level Maths with Oxbridge, I am happy to share that.