Even if the deeds of the secret services are fully secret (a big if), your argument is still incorrect. Assassinations of heads of state are rare and unusual events, and are normally investigated thoroughly. It may be that in every such assassination prior to 1939, the evidence points towards culprits other than the British secret services.

(Whether or not this is actually the case is another question; I am merely demonstrating that your argument doesn't work even if its assumptions are fully granted.)