Court uniform and dress in the United Kingdom
Court uniform and dress in the United Kingdom were worn by those in attendance at the royal court up until the mid-20th century and are still worn as formal dress by certain office-holders. Specifically, court uniform was worn by those holding particular offices associated with the Crown (including certain specified civil servants and members of the Royal Household, and all Privy Counsellors).
Source: Wikipedia — Court uniform and dress in the United Kingdom (CC BY-SA 4.0)