We do indeed have a right to refuse an ILLEGAL order - in fact we have a moral and legal responsibility as officers to do precisely that.
The difficulty is determining precisely what consitutes an ILLEGAL order.
In the case of Iraq, post UNSCR 1483, orders to serve in Iraq are most certainly LEGAL (as the SCR legally recognizes the occupying powers). - So Doc appears to be sh*t out of luck....
Iraq, pre-1483 -- more difficult. My interpretation would be that the Attorney General's legal advice to HMG (whether politicised or not - debate all you like) was that use of military force WAS legal, therefore you are on a sticky wicket when it comes to refusal.
The legality of orders is usually more straightforward when it comes to the decision to go to war, it becomes fraught with difficulty during the conduct of said war (most LOAC governs behaviour during war).
The bottom line is that intelligent thinking officers must endeavour to ascertain the legality of orders to the best of their knowledge (huge caveat) under all circumstances.
Blind obedience will not help you when you end up in front of the Hague.....It didn't work for the Nazis at Nuremburg, nor should it have done.