Yesterday, HM the Queen marked the start of GCHQ's centenary celebrations. In 1919, the Royal Navy's Room 40, of which I write in my second and third novels, became part of what was then known under the cover name of the Government Code and Cypher School. The official date for its formation was 1 November 1919, but of course, Room 40 was established in August 1914. The Queen unveiled a plaque to commemorate the centenary and containing a coded message. I have decoded it and added the answer to the end of this post.
The Royal Navy continued to run the GC&CS until 1922 when it was handed over to Foreign Office control. Even so, it was headed by a naval commander, Alastair Denniston, even after the move to Bletchley Park, and the RN still provided administrative support (hence the number of WRNS personnel operating the bomb machines). Moreover, when the government would not put up the money to buy Bletchley Park, Admiral Sir Hugh Sinclair, the former Director of Naval Intelligence and prevailing Head of MI6, bought the site with his own money.
In 1946, the GC&CS was renamed as GCHQ and moved to Cheltenham in the 1950s. Of interest, my wife's late-uncle, Ron Challen, features on the GCHQ website. https://www.gchq.gov.uk/features/story-signals-intelligence-1914-2014
Ron had many fascinating tales to tell of his experiences, but only in his later years.
The decoded answer is, "1 Hundred".