The series is set in 2013. London has been evacuated after a nuclear attack, which is why MI5 has to set up field offices all over the United Kingdom. 'Code 9' will focus on six young spies who have been recruited to help set up Field Office 19. Among the members of the team are a mathematics whiz, a former police officer, a criminal gone straight, a medical intern, and a psychology major.
Code 9... Field Office 19... I am not a number, I'm a free man!
The explosion has changed the political landscape and a lot of the upper echelon at MI5 must be gone. According to producer Chris Fry, "MI5 needed new officers with ability, dedication and potential on the streets as soon as possible. With the exception of their team leader, none of them have been in the field for more than four months and they will continually find themselves in situations beyond their experience and training - they'll only succeed through grit and determination."
I wonder if they will spell out when the nuclear event took place? My bet would be on the 2012 Olympics, at some point after the departure of the Doctor and Rose in the 'Doctor Who' episode "Fear Her".
Since this would cause a seismic change in daily life in Great Britain (if not the rest of the world), then 'Code 9' would have to be shunted off to its own dimension as soon as the explosion takes place. Otherwise it would cause too many problems for other TV shows that should be sharing the same dimension. "Why is London a pile of devastated ruins and yet they're still serving drinks in the Queen Vic?" (Come on, you KNOW 'EastEnders' will still be running by then!)
The current series of 'Spooks' would remain in the world of Earth Prime-Time; there's no reason to move it. It stays abreast of current events that are happening in the real world without too much deviation that can't be absorbed or ignored in other shows. And the world of 'Code 9' will share that same timeline until the nuclear explosion when it branches off into its own world.
But 'Code 9' doesn't have to reside in that TV dimension all by its lonesome. I think it would work if it shared the space with the TV series 'Jack And Bobby' from a few years back. Just because none of the talking heads from their future timeline ever mentioned the nuclear explosion in London, that doesn't mean it didn't happen. By their point in Time, it was long over and had nothing to do with the Presidency of Robert McCallister.