(Posted in 2 parts):
When an application form simply asks if the applicant has a criminal record, the employer has to trust that the applicant is telling the truth. However, if the applicant gets the job and the employer subsequently finds out that the person has a criminal record (excluding any 'spent' offences which he was not required to declare), this would provide grounds for immediate dismissal. Also, if the employer reported the matter to the police, the (ex-)employee could be prosecuted for 'obtaining a pecuniary advantage by deception'.
Where an employer is not simply prepared to trust that an applicant will tell the truth, he can apply to the Criminal Records Bureau to check on the applicant's background. In order to stop 'nosey' people from pretending to be an employer, the CRB only accept applications from registered employers or registered umbrella bodies.
The 'registered employers' system allows large bodies (such as health authorities), who require over 100 CRB checks per year, to satisfy the CRB of their integrity. (i.e. to show that they have a legitimate reason for making the checks and that they'll only make checks on their existing staff or on job applicants).