Has the British Government in the past killed people by "unfair means"? - including British citizens? - probably Yes (David Kelly?) Has the British Government in the past forced someone into "social isolation" (and eventual suicide) for their sexuality? - probably Yes (Alan Turing?) Has the British Government gone to war on false pretenses? - Again, probably yes (Iraq) Did the security services keep tabs on the leaders of the mining unions during the strike? - Yes Has the British Government and its allies used the services of terrorist organizations to destabilize the governments of foreign countries? - Again yes (Iraq, Syria, etc, etc). Is it unreasonable to suggest that the British Government or the security services could try to arrange an assassination through a terrorist organisation? Is it unreasonable to suggest that the British Government could assassinate someone and fabricate the evidence to frame them as a terrorist? I'm not saying they did in this case, but I am suggesting that they have previous form for such acts and it is not behind the wit of man to see that they could have done it.
Absolutely... is this not the case of all government's, democratic or un-elected? Enemies of the state and all that. It's the comedy letter used by the OP that's the problem. He could and should have used a more plausable source of 'documentary evidence'.