Is it dangerous for British and/or American tourists to go to North Korea ?

I don't think they allow tourists. There ARE Brits and Americans there- there's a school run by Americans, for example. It's not dangerous in the sense of a random person killing you, but it is dangerous if you don't obey the laws there. There was a guy arrested for taking down a poster a few years ago who died in the North Korean prison. 

It's dangerous for anybody to travel there because the North Korean government does not guarantee the safety of foreign nationals. Obviously British and American tourists in particular are looked upon with a greater sense of suspicion than tourists from many other countries, but I'm not sure that would equate to it being more dangerous for them, though the current political climate between the DPRK and the West could certainly be a factor for visitors so it would be advisable to go during a time when things are relatively calm rather than during a period when relations are more tumultuous.