And the terrorism alert status hasn't risen a notch? Pull the other one.
Anyway, to the point: this wasn't simply an assassination. There are any number of poisons out there that would do the job painfully well but much more rapidly, and without the same scope for a diplomatic incident. Likewise, a bullet to the back of the head would have worked just as well (as witness the assassination of Anna Politkovskaya).
What this is, is a warning: "we have the capability to detonate a dirty bomb in central London any time we feel like it, so don't fuck with us". (Just take Polonium and add a little TNT.) ..."