Williams was unhappy with his work environment at MI6 and felt he didn't fit in with his colleagues. During the inquest, testimony revealed that the coder had conducted unauthorised searches of an MI6 database
also: http://www.wired.com/threatlevel/2010/08/codebreaker-death/
"The source indicated that Williams' work to disrupt the Russian mafia could have put him at risk".
"Some of these powerful criminal networks have links with, and employ, former KGB agents who can track down people like Williams," the source said.
If you assume the British government chooses to murder people, it also perfectly reasonable to assume that it does it using someone else style to cover its crime e.g. russian mafia style.
EDIT: To expand on this now that I'm not mobile: This notion of the "style" of security agencies might be what you get from Hollywood and spy novels, but you could only arrive upon this conclusion if you are selectively excluding examples from reality. Yeah, we know of the US's "heart attack gun", but we also know they are rather fond of "assassination via drone" and don't give a shit who knows it. The Russians tried to sneak the Georgi Markov assassination under the radar, but killed Alexander Litvinenko in one of the more flamboyant ways imaginable. Mossad tried to be sly with Mahmoud Al-Mabhouh, but they've blown other peoples' heads off with telephone bombs on more than one occasion.
The conclusion that should be drawn from real-world examples of state assassinations should not be one that you find surprising: they do whatever they think will work best in any particular situation. That is their style.