Just to add to the above:
Unless the type of employment is one that's exempted from the provisions of the Rehabilitation of Offenders Act (e.g. working with children), the only type of DBS check available to you wil be a 'basic' one. Such checks exclude all 'spent' convictions, so a job appicant could have been convicted of many crimes several years ago without anything showing up in a 'basic' DBS check. (If the offender was only fined, a conviction would become 'spent' after just one year).
35% of all British males acquire a criminal record before their 30th birthday, so a very large number of job applicants will have criminal records. However most of those applicants will not be required to disclose their records because their convictions have all become 'spent' (and any 'basic' DBS checks will show nothing recorded).
If a potential employer asks a direct question (either on an application form or in an interview) such as "Have you ever been convicted of a criminal offence?", a person whose offences are now spent can lawfully answer 'No'.