This article in Asia Times
gives some details about the position of our government on assassination. It makes clear that our govenment are not allowed to carry out assassinations within the UK but does not mention something I once saw alleged on a US newspaper story - that since the CIA are not allowed to do things on US soil & SIS on UK soil they sometimes act for the other. Since British & US intelligence have an almost Siamese twin relationship this seems not improbable. The article ends
Contacts within the Intelligence communities both in the UK and the US strongly advised AFI Research not to run this piece on assassination. However, we consider that such a response merely gave added credence to the suggestion that in the future the British authorities may indeed be prepared to use more "positive" methods, under certain circumstances, in dealing with both external and very probably, internal "enemies of the state". It now seem almost certain, therefore, that a limited number of selected and highly trained MI6 officers have once again been given a "license to kill", and perhaps very largely because of the experience and influence of the Israeli secret service.
(AFI Research, a leading source of specialist intelligence, defense, terrorism, conflict and political analysis)
In full http://www.atimes.com/atimes/Middle_East/EF13Ak03.html