In his statement to Parliament, the Prime Minister said that the government acted in defence of the United Kingdom when the drone strike was ordered against Reyaad Khan in Syria - please also see post of 9th September. The U.N. Charter Article 51 requires that such self-defensive actions be reported immediately to the United Nations and here is a link to the UK's letter:
Letter dated 7 September 2015 from the Permanent Representative of the United Kingdom of Great Britain and Northern Ireland to the President of the Security Council
I don't know whether a delay of over 2 weeks constitutes an "immediate" report but we can be certain that this point will not affect the legality of the U.K.'s action.
The letter refers not only to defence of the U.K. but also to the defence of Iraq and Iraq was not referred to in the Prime Minister's statement. Politically this seems to be a considerable mistake since an impression is created that government thinks it necessary to bolster its case (that the action was lawful) by referring to Iraq. Perhaps an alternative way of looking at this is that the UK government is reminding the Security Council of the fact that UK Forces (and others) are already acting against Islamic State in Iraq.
Whatever the political situation, it is hard to see why it should alter the legal position. If the aerial attack was lawful in self-defence of the UK then it will remain lawful on that basis regardless of the position vis-a-vis Iraq. As argued in this previous post, the action appears to be supportable in international law though this will depend on ALL the facts and many (maybe most) of those are not in the public domain.