Special leave is granted in certain circumstances, subject to the work requirements of the civil service. In certain other circumstances, the head of department or office can grant special leave without pay at their discretion.
You should note that unpaid leave does not count towards pension calculations. The approvals process varies according to the type of special leave you want to take.
You need to seek approval by your manager for: ante-natal and post-natal classes, bereavement leave, interview leave, jury service/witness in court leave, leave in lieu, marriage/civil partnership leave, union business leave, or special paid leave.
You need to obtain approval for leave from the local HR team when you want to: accompany a spouse abroad; take leave due to domestic or family reasons or because of force majeure; to take part in a charitable organisation, Civil Defence, or the Reserve Defence Forces; to work with the EU or an international organization; to take special leave for sporting purposes; or to take paid leave as a living kidney donor.
Please read this page, which explains the process for each type of leave in turn.