I've been watching more than my fair share of BBC shows on DVD lately--Extras, Torchwood, and Life on Mars in particular. These range in quality from brilliant, decent enough if you ignore the first season, and thoroughly enjoyable, respectively. The 2000s were clearly a good decade for television on both sides of the Atlantic. That said, there's one crucial difference between the best shows in the US and the UK, as far as I can tell. The kinds of people who get starring roles in British television are markedly different from the people who star in American shows: they look like real people.
Watch, for example, Ashley Jensen's brilliant work as dim-witted actress Maggie Jacobs on Extras. Jensen's deft touch keeps Maggie from being the kind of stock "village idiot" sitcom character that the show itself lampoons, and adds a particular sting to the awkward humor. It's the kind of role that very few people could pull off with such charm, and really should have led to a wealth of future lead roles for Jensen. Maybe on British TV it will, but here it got her a bit part on Ugly Betty, perhaps because she's neither outrageously thin or glamour-model pretty.
Or compare the casts of the UK and US versions of The Office. The remake features a lot more variety in casting than most American television (and kudos for that), but the leads have still been assigned to thin, conventionally-attractive people. John Krasinski is a great, funny actor, but it's still hard sometimes not to see him as a bizarro-world Martin Freeman, and just as difficult to picture someone who looks like John Krasinski being stuck in a dead-end paper company job. Slate's Seth Stevenson gets to the heart of this when reviewing the remake of Life on Mars, noting that the cast in general is better-looking and better-known than the original--and that the new casting completely undermines the show's interpersonal dynamic. Even within genres, this holds true: there's not a single person on the entire cast of Torchwood who's as sexy as the least-attractive Galactica crew member, and while the latter is a better show, it's still kind of hard to understand how the ragtag fleet maintains such flawless fitness and perfect skin on a diet of algae and moonshine.
Why the difference? Is it that the larger pool of American talent makes it easier to find people who are both talented and blandly good-looking? Is it some kind of institutional mandate brought on by publicly-funded media? Ultimately, who cares? Diverse casting isn't a magic bullet, and there are still plenty of BBC programs I find unwatchable (confession: The IT Crowd bores the crap out of me). But there are certainly a lot of cases where it makes a show better (including many American shows: The Office, The Wire, and 30 Rock come to mind), and it's got to be healthier for the viewing audience.