The royal family and Department for Culture, Media and Sport do not have a public-facing policy on this to quote, so the following is taken from the British Flag Institute. The guidance starts by ...

Vexillology is the study of flags and related emblems ... the Second Sea Lord. The Royal Navy spokesman added the British White ensign, which is only used on UK warships, "cannot be defaced ...

The National Museum of the Royal Navy (NMRN ... "The sledge flag, one of the earliest known in existence, helps to tell the story of British obsession in the 19th Century with arctic exploration ...