From the murderer's, standpoint, it would be better to have him vanish and then just have speculation and conspiracy theories about why than to have enough clues to remove doubt and lead investigators to the murderer(s).
We'll have to wait and hear more about this story before coming to conclusions. It truly may just be a case of a man have a heart attack while exercising.
I do not think the UK government would have killed him in such a public manner. It would only increase the outcry.
However, someone who has a vested interest in embarassing the Blair government would do so. Since I don't think Ian Duncan Smith would do this, I am left with Iraqi or French agents, minions of the anti-Bush faction here in the USA (most especially the Clintonistas) or the radical leftist denizens of the BBC themselves.