References are usually taken after an offer is made, often as a condition of it, and may be written or given by phone. Many UK employers only confirm factual details, such as job title and dates of employment, as a matter of policy. Where a fuller reference is given, a set list of questions, for example on reliability, reasons for leaving and whether the referee would rehire, makes answers easier to compare.
Reference checks work best as a final confirmation rather than a deciding factor, because referees are chosen by the candidate and tend to be positive. Asking every referee the same job-relevant questions, getting the candidate's consent before contact and handling the information under data protection rules all help. A reference that raises a concern is a prompt for a conversation with the candidate, not an automatic outcome.