Soldiers outside a war zone are not legitimate targets (hence the dubious legality over the sinking of the Belgrano in the Falkland's War for example). And furthermore the Taliban aren't, in international law terms, a legitimate state so they can't really have legitimate targets. By international law we're not actually invading or occupying anywhere, we're in Afghanistan and other places with the express permission of their recognised governments. Furthermore, this was an act of terror rather than war since the aim clearly wasn't to remove military personnel who were threatening Afghan civilians. They were just killing a guy as a publicity stunt - both sickening, and stupidly and excessively counterproductive. Finally, as you point out, they were UK citizens so can't make an act of war on their own country. That's sort of the point of this democracy thing we have; the people of this country have a say in who runs the country, and they in turn control whether or not we're at war.
Muslims against Crusades was an organisation which believed Muslims are "not obliged to obey the law of the land in whatever country they reside". They were barely peaceful, and set out very deliberately to act in an inflammatory manner. People can believe what they like about what our government is doing to Muslims, but regardless of the intended ends the means cannot be sympathised with or justified in any sense to my mind.
