Ugandan officials are claiming (without much evidence, to be sure) that Al Qaeda was plotting to kill Queen Elizabeth at a Commonwealth meeting in Kampala last year.
Whether or not it's true, it's certainly plausible, and would have been exemplary of global guerrillas' search of what John Robb refers to as "cultural systempunkten": that is, attacks that activate a deep-seated cultural faultline, causing "irrational" counterresponses. The most conspicuous recent example of such an intentional act of cultural provocation was the bombing of the Golden Mosque in Samarra in February 2006, which by enraging the Shi'ites, precipitated the worst period of communal bloodletting in Iraq since the start of the war. The point of such attacks is that they cause systems disruption in an indirect fashion.
Had it been successful, the Qaeda attack on Queen Elizabeth in itself would have had few if any direct implications for the functioning of either the international system or the British state or monarchy. Queen Elizabeth herself, personally, is unimportant. However, such an attack might well (and certainly would have been intended to) provoke cataclysmic iatrogenic effects from the British, of a sort that are relatively easy to imagine. Depending on leadership's reaction, would not the British public have been roused, albeit in unpredictable ways? A successful assassination might, on the one hand, have precipitated massive demand for withdrawal from Iraq/Afghanistan. Then again, it might also have sparked retaliatory lynchings of Muslims in British slums (to be broadcast live by Jazeera, natch), with attendant roiling of global affairs and improved recruitment opportunities for Qaeda, etc.