Well, great question, but I suspect for many a no brainer. The oath of allegiance we all swear when we enter our Armed Forces is to our Nation, and not to a political party, or government of that particular time. The embodiment of our nation is HM the Queen, or whoever serves the country as Monarch, and yes, I use the term "Serves as Monarch" deliberately, because, despite wealth and entitlement our royal family have given Great Britain stable rule and a unifying influence for many years, and all serve, as do our servicemen. It's been a long time since I was sworn in, I can guarantee that an insistence on swearing allegiance to a politician would have radically foreshortened my career. HM and her family might not sit well with the EU brigade, I'm afraid that old fashioned people like myself still keep our trust in what we know and are comfortable with.
Smudge