There is a reason for this: so that the Army (or any of the other British institutions that nominally have the Queen at the top of their org charts) can't be politicized. That's the theory anyway, tho' New Labour tried their best to stuff the civil service and the BBC with their cronies. So when the PM of the day says "go and shoot the leader of the opposition" the soldier or marine or policeman can say "sorry mate, I actually don't work for you".
As far as I can tell, the status of the armed forces in the UK is rather complex - they do indeed report to the monarch in her/his capacity as commander-in-chief but they only legally exists because of Parliament repeatedly passing Armed Forces Acts every 5 years or so to legalize them otherwise they would be illegal under the 1689 Bill of Rights:
Also worth noting that we don't have a Royal Army:
"Our Army is not called the Royal Army (unlike the Royal Navy and Royal Air Force) because, after a historic struggle between Parliament and monarchy, the British Army has always been answerable to Parliament and the British people. If you live in Britain today, the actions of the British Army have affected the culture, traditions, government and laws of the society you live in, and, on a global scale, are continuing to do so today."
Which I take as a reference to Parliaments New Model Army that won the English Civil War against the monarch:
Having said that, Army officers that I have known tend (in private at least) to be somewhat skeptical about politicians of all parties and very clear that their ultimate allegiance is to the Monarch rather than the PM - given that PMs we've had in the last 15 years or so I'm rather glad of this.
If the English Civil War, the New Model Army and the struggles between the monarchy and Parliament are of interest to anyone, Mike Duncan (of "The History of Rome" fame) has recently wrapped up the first segment of his new "Revolutions" podcast[1] on the English civil wars of the 17th Century.
ahem Northern Ireland? The UK army has a history there of using armed force against political opposition. Start reading about Bloody Sunday or internment without trail ( http://en.wikipedia.org/wiki/Bloody_Sunday_(1972) ) and onwards.
