UK I would think seconds. There is so much survellience, so few "dark areas" and loyalty is so high the groups of people that they would trade family members for country. Its a small island so getting anything, anyone on or off without detection would be improbable. Your going to write yourself into dead ends. Unless you can trick them into it its not going to sound believable.Easier to just hijack a truck based ICBM in the middle of someplace quiet in Russia - there's parts of that country you could have a war and no-one would notice for a few days. In the UK, you'd be pretty busy in minutes.
Ian
You would have more luck on the continent. Perhaps weapons based in germany (US, UK, French or something) where you could perhaps write in a some sort of confusion and dozen different ways to disapear. Eastern countries offer so many more possibilities. There are quite possibility soviet stuff out there that has been overlooked or at least poorly secured. There are plenty of states that don't like Russia and/or Russia doesn't particularly like to make it seem more believable. Then using ex-uk forces would sound more believable, heck they could even be undercover agents, perhaps one or two could be tripple agents.
That would be more believable than ~5% of the ex UK SF defecting to turn on their comrads and their country to secure a dangerous weapon to dodgey people and no one leaking it or shooting all those involved with western weapons in one of the most difficult countries to operate in.