The BBC is distributed in the US by Warner, as outside the UK they're a commercial enterprise like everyone else. In the UK, they're a public organisation that has an obligation to support any format that's commercially viable (I forget the exact wording), so they'll probably support HD DVD here as long as people are buying it.
A few more of their shows are out on both HD formats here. None of the really big ones that I'd want (Torchwood, Doctor Who, etc) but the first season of Robin Hood is out (
BD and
HD), as are a few of their miniseries' like Bleak House (
BD and
HD). They don't promote them at all so I don't know how they expect them to sell or people to find out when they're out, though.