Not necessarily (since 1908) Keith although until I was sent this reference a couple of years ago I would have agreed with you."From its earliest days, the Admiralty often referred to the flag – however it was used – as the Union Jack. In 1902 an Admiralty Circular announced that either name could be used officially. And in 1908 the UK Parliament approved this verdict, stating that ‘the Union Jack should be regarded as the National flag’."From https://www.flaginstitute.org/wp/british-flags-2/the-union-jack-or-the-union-flag/
N V Brooks ● 1772d