On the question of using nerve gas for assassination I quote from Spycatcher (Wright 1987, 16). “At the beginning of the Suez Crisis, MI6 developed a plan through the London station, to assassinate Nasser using nerve gas. Eden (the PM)  initially gave his approval to the operation, but later rescinded it when he got agreement from the French and Israelis to engage in joint military action.” When that invasion failed the plan was resurrected but according to Peter Wright then failed ‘lamentably’.