Uh, most British series are just a six episode order (Spooks went to ten episode orders from its second series on, a relative aboration in British television). Still, it's a fair point. The British serial State of Play was incredible, but I would be hard pressed to see it work as a continuing series (though I haven't seen the American feature film remake, I don't expect it to work as well in that format, either).
I like David Simon's plan for The Wire. Ideas for five seasons, but each year is (more or less) a self-contained narrative that could serve as a proper end to the series if need be. But HBO offered a somewhat unique environment for that kind of storytelling. That just doesn't work on network television. Think of Prison Break, a planned two season series that was extended to four painfully thin seasons.