"UK forces are present in Iraq at the request of the duly elected Government of Iraq "

German forces were tramping all over Europe at the bequest of a duly elected German Government... :-)

"Their presence is therefore perfectly legal"

ditto.... legally present during a legal war of invasion that is...
:-) so any attempt to refuse an order is just the same as this case...

In France, the Germans were there with the consent of the 'puupet Frech Govt" :-) just as is the case now in Iraq, execpt the puppet is currently throwing a tantrum there... :)