There never was an explanation for them only appearing in the UK, other than, I guess, limited knowledge of them. Of course budgetary reasons will have played in to that, though there was occasionally mention of them being outside of the south east, the north sea and in Wales I seem to remember.
But I guess there needs to be no other reason than it's filmed in the UK therefore everything happens here, just like everything happening in Sunnydale in Buffy and in Smallville or Metropolis in Smallville. That kind of thing.