I would like to know whether in law Napoleon was a prisoner of war when he surrendered himself to the British after the Battle of Waterloo, did he retain that status up to the point when he was landed at St Helena and did his legal status change at St Helena because it belonged to the East India Company rather than the British government?
Napoleon surrendered himself to the HMS Bellerophon on 15 July 1815. He was transported to the southern coast of England and eventually set sail for his imprisonment at St Helena on HMS Northumberland on 7 August. He reached St Helena on 1 October.
I think two treaties are relevant. First, the Convention of St Cloud was signed on 7 July 1815. This signified the end of hostilities and allowed the British and Prussian troops to freely enter Paris. Napoleon surrendered himself after this date.
More importantly (I think), the Treaty of Paris was signed by Britain and France on 20 November 1815. Article 10 of this stated: “All Prisoners taken during the hostilities, as well as all hostages which may have been carried off or given, shall be restored in the shortest time possible.”
A search of news stories in the London Times suggests in practice the British government did not start releasing French prisoners until late December 1815: “Government gave directions in the course of last week, of the agents of the prison depots at Forton and Dartmoor (Captains MOTTLEY and SHORTLAND) to provide vessels and send home all the French prisoners that are under their charge. Yesterday those in confinement at Forton commenced being shipped, either for Havre or Cherburgh". (The Times, 26 December 1815, p 3).
The relevance of all this is that Napoleon arrived at St Helena on 1 October, BEFORE the Treaty of Paris was signed. Since St Helena belonged to the East India Company, I suspect the island lay outside the jurisdiction of British courts AFTER the Treaty was signed (regarding the EIC's independent powers of legislation, especially pages 19 and 20). In other words, once he had been taken there, the British authorities were no longer obliged to release him according to the terms of the Treaty of Paris. It may be that that this was an important motive for sending Napoleon to St Helena, not just because it happened to be very isolated.