Regrettably, probably at the beginning. There don't see to be any 'short-cut' routes from nursing to becoming a doctor. You'll still need to do a five-year university degree course + postgraduate training. The entry requirements are normally an absolute minimum of an A and 2 B's at A-level. (The most popular medical schools might require 3, or even 4, grade A's). These normally need to be in mathematics and science subjects. Candidates without a science background can be accepted but the university course is then extended to 6 years. If you've already got a 'good' degree in a relevant subject then a 4 year course is available. The only credit which your nursing experience and qualifications would be likely to bring you is that, possibly, a university might be prepared to accept (very) slightly lower grades at A-level.
Further information, with some relevant links, is here:
http://www.learndirect-advice.co.uk/helpwithyourcareer/j obprofiles/profiles/profile691/
Chris