How quickly must I report a new director or a new shareholder?

Director appointments, resignations and changes to their details go to Companies House within 14 days, and so do changes to people with significant control. New shares must be reported within a month of issue. A new director gives their personal code as part of the appointment filing, so they should verify their identity first.

Corporate changes

Source: Running a limited company: company changes you must report, GOV.UK