@panzade,
That would be MI6, not M16. However, that could be considered a reliable source, depending on what references the authors give for their claims. Certainly there were many incidents in the war of planned assassinations. Acting on information from Enigma intercepts provided by the English, the Czech underground planned an ambush, and successfully assassinated Reinhard Heydrich in 1942. As chairman of the Wannsee Conference, he can be considered the architect of the "final solution."
Even before the United States had entered the war, ONI (Office of Naval Intelligence) had broken the Imperial Navy's code system, which allowed them to crack each new version of the code within weeks of it being implemented. This enabled them, for example, to learn of the Midway operation before Nagumo's fleet arrived. The use of these "Magic" intercepts allowed the Navy to construct the itinerary of Admiral Yamamoto in 1943, and they set up an ambush with P38 fighter aircraft to attack his air convoy as it left Rabaul. It was successful, and Yamamoto was killed on April 18, 1943.
Even if the source you provided is reliable, the reason they give for not attempting the assassination is mistrust of the Germans with whom they might have worked. It does not support a claim that they declined to do so "for the war effort."