Were the prominent Russians in the UK who were killed by plutonium or chemical warfare agents actually killed by the CIA and/or the SSP working at their behest, as a disinformation campaign to stir up anger towards Russia?

These indeed were examples of false-flag operations. It is routine for operatives of the CIA to do targeted killings that can be blamed on foreign powers and then stir up tensions and help to destabilize international relations. Creating a history of such events keeps the taint of evil in place to tarnish the image of other nations and their leaders, and this creates an acceptance and willingness of the general public to agree to the need for war when things reach a fever pitch, because so many examples of their evil have seemingly occurred. And it is easy then, for the public to not see them as human with full rights and this serves the Extraterrestrial Alliance to carry out all kinds of war-making with support of the public, who pay for the expense involved with their tax dollars and with their lives.