UK TV doesn't keep the same production schedule as US TV, you'd have a hard time making more, it takes them 9 months to make the 13/14 they do now. Why sacrifice the quality for quantity?Well, for me 13 episodes is just too short. I'm not sure how a show can have 6 episodes a season, unless its a mini-series. With such a world wide demand for the show one would think that they would bump up the number of episodes.
Oh well, I can survive without it, but the short run risks that new fans, younger fans, forget about the show and move on. At least here in the US.