Do secret service agents within the UK have a license to kill?  

It is true that within the 4 (yes, we actually have 4, and the MI&CI branches number from 0-13, although we usually only hear of 5&6) UK intelligence agencies, there is no such thing as a 'license to kill', though, any operative above a certain level, knows how to kill, and would do so if ordered. I believe that Dr Kelly committed suicide, and that this was because he had lost so much by his exposure, and had been an exceptionally driven and proud man, who merited everything he had achieved. If the intelligence services had been involved, they would have sent him back to work in some dodgy neck of the woods abroad, and killed him there, framing some local political or terrorist organisation. Killing him within the UK taised his profile to an even greater public height, and prevented the story from fading into oblivion. It would therefore have been detrimental to the interests of their paymasters.