It's a mixture of both, to be honest.
First there's a wider cultural difference between UK and US (UK double, single internaly v UK single, double internally). But then you have publisher differences where some UK publishers prefer the American version, some the British.
Every publisher will have their own house style. Eg, I'm UK too, but I edit for an American publisher, so I edit to the double speech marks for them. I also write for an American publisher who likes an English feel, but they still prefer the double speech mark usage.
I've got to admit, I barely use the single system becuase of that. Eg, I don't write:
[']How are you?['] (single)
[']Huh? ["]How are
you?["]['] He scowled. [']What the hell do you mean by that?['] ('How are you' in the second example has single, then double style.)
I'll edit and write:
["]How are you?["] (double)
["]Huh? [']How are
you?[']["] He scowled. "What the hell do you mean by that?"
If ever you're in doubt, you can ask the publisher you're targetting, or just look at some novels they've published; that will tell you which style they prefer.