the British government wasn't that stupid, was it? Killing Boney - just what would have done it? More important for them was to win the war - and you won't win a war by just the assasination of a general, as good as he was.
In that time period France had still quite a talented officers pool at the highest level to re place Boney with ease - and moreover they would be powered by a much greater moral strength than Boney ever was, they would fight for la patrie.
This they successfully did in the French Revolutionary Wars, amongst else, against British paid and equipped emigré units.
They could have done it - much better than Boney in 1800 and later, there they wouldn't have had any megalomaniac ideas to rule the world.