Bednobs has provided a full and accurate answer to your question (i.e, the CRB does not accept requests from individuals).
However, if you want to have a document to present to potential employers, you can obtain a copy of the information held about you on the Police National Computer. This will show all convictions (or, of course, the lack of them). You simply need to write to your local police HQ and pay a fee of �10. The following link relates to the Metropolitan Police but you can adapt the PNC request form to use as your request to any other police force:
http://www.met.police.uk/dataprotection/forms. htm
Chris