National Insurance numbers have nothing to do with British citizenship. (In the tax year 2005/06 alone, 662000 National Insurance numbers were issued to foreign nationals).
You will still have the same NI number that you were issued with as you approached your 16th birthday. (It doesn't change and it doesn't require renewing). If you've no longer got a record of what it is, your local HMRC office can assist you:
http://www.hmrc.gov.uk/local/individuals/index .htm
Further info:
http://www.hmrc.gov.uk/faqs/ynino.htm
Chris