This is really important: You only tell if asked. For example, on an application form or during the interview.
If you are applying for a job which involves any of the following, you will have to undertake a DBS check (this used to be called a CRB check):
You will know you are going to be subject to this check because you will be asked to sign a declaration at the application stage. If you are successful at the interview you will have to complete a DBS form and provide a range of identification.
The employer can’t fill out this form without your consent.