When HM Forces salute the respect and courtesy is ultimately being shown to or for HM The Queen. Her Majesty confers and delegates this honour on named individuals through commissioning scrolls, governorships, Secretaries of State, some Lords & Members of Parliament and even a few bishops if they have been appointed to the Privy Council.
Incidentally, Diane Abbott was appointed to the Privy Council in Feb 2017, so quite a high bar.