Some employers will want to see one when you apply, most will expect it to be obtained after the offer of the job but before the post is taken up. The offer will be conditional on the Disclosure and Barring services check. Its not called CRB anymore. Have a look at this.
It used ( ha! ha! ) to be stated that only the lucky winner of the interview would have a CRB but it became obvious within a few months that a machine-gun approach was being used