It is now a criminal offence for employers to require job applicants or existing employees to obtain a copy of their criminal records, known as making an enforced subject access request. Obtaining details of criminal records in this way can lead to disclosure of spent convictions and is punishable by an unlimited fine. The correct approach is for employers to use the criminal records disclosure regime operated by the Disclosure and Barring Service (DBS). The DBS operates different levels of disclosure depending on the nature of the job.
For more information, see the Information Commissioner's Guide here.