Wait, so what exactly is the British government style of killing people?
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.
The style of a security agency generally would be to make it look like either an accidental death (car crash) or a natural death (exotic virus), so as not to attract the attention of the police or the press. The mafia style, on the other hand, would be to make it clear that he had been executed, so as to send a threatening message to the appropriate circles.
I wonder if state apologists on Russian web forums say the same thing whenever Alexander Litvinenko is brought up.
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.
Actually, from the Mossad example, many of the more "known" Mossad deaths have been from bombs. So you can say that at least some spy agencies have a pattern.
The general practice is to leave known spies operating and use them for counterintelligence. No other explanation is consistent with the small number of prosecutions and premature deaths.
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.