sorry, i know the one you mean and i like it too, i tried looking but all that came up was doctor who and blue peter, at the end of the show watch the end credits and it will tell you who sings the theme tune.

EDIT ive foud it... theme tune to the BBC series South Pacific? Israel Kamakawiwo'ole - 'Somewhere Over The Rainbow' http://www.google.co.uk/search?q=bbc+them+tunes+south+pacific&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-GB:official&client=firefox-a