This is necessarily going to be a very short answer, so hopefully the mods will be merciful.

MI6/SIS never declassifies material, as a matter of policy. The only historian to ever be given access to SIS archives was Keith Jefferey, the author of the official history book of SIS. He was given unrestricted access pertaining to SIS activities between 1909 and 1949. The only mention of assassination in the book is of the question of whether to designate targets for SOE missions after D-Day. SIS decided not to get involved, but not to discourage resistance fighters from their assassinations.

Unless the policy of over a century is overturned, we will never know much about SIS or its history.

However, a Licence to Kill is essentially redundant for SIS, as it could only permit killing within Parliament's jurisdiction (the UK). SIS exclusively operates outside that jurisdiction, so a Licence to Kill would be meaningless.

That said, a former "C" testified in a public hearing (sorry about using the Mail) gave evidence that any foreign laws could be broken by SIS operatives if issued a Class Seven authorisation in writing from the Foreign Secretary. This would make the operative immune from deportation or prosecution by the British courts. In theory this extends to lethal force, but there has never been a confirmed case of this happening.