Jack Valentine is an agent with British Intelligence.
The agency for which he works at the beginning of the series and for which he has served for more than fifteen years is called the MRU, an apparent acronym for which no breakout is disclosed. The MRU is an ultra-secret agency of a type known as shadowy.
Under the concept that no agency could be legally allowed to do the things that the MRU must do, it cannot actually exist and so it doesn't. And the numerous employees of this agency that doesn't exist don't work for it, either. Nevertheless, the non-existent agency is there for the bidding of the Prime Minister, doing what the others like Mi-5, MI-6, and such cannot. Its area of activity is virtually unlimited and it could call upon the other agencies for help but that would not fit with the fiercely independent air that the MRU has.
Valentine's cover during the period of his non-employment for MRU is that of a travel reporter, going to exotic locations and writing about the great things to see and do. He actually has not been to many of the locales of which he has written and certainly hasn't written about any of them but the cover has done its job on several occasions. His reputation is fairly established as a good solid investigator who can handle himself anywhere, whether one is talking about Valentine as a reporter or as a spy.
As the series begins, Valentine is growing tired of the action and is considering retiring. He has certainly paid his dues and is owed some peace and quiet, but the action doesn't stop and neither does Valentine.