I think the better question is, will you be accepted to a top-tier British university without taking either the O-level or A-level exams? Generally, not. Colleges at Oxford or Cambridge will require you to take a number of A-level exams in different disciplines for entrance. In addition, you will need to interview and submit a paper to the faculty. This same requirement is held for foreign applicants, and is also the case with the better universities outside of the Oxbridge system.
I would recommend you finish up at a British boarding school and start preparing for the exams. Alternatively, you could do a post-grad year at a private school. Finally, there is an option to enter some universities through a Pathways program, which helps foreign students catch up.
I did my graduate degree at King's College, Cambridge.
But, in general, it will be pretty impossible to bypass entrance exams in the UK.