toss a proxy past your exit and you can restrict it any way you want.
That breaks the Tor protocol. It means that non-bbc websites would appear to be blocked and unblocked at random as users used Tor.
I had a discussion with one of the developers and basically Tor isn't designed to do this -- the blocks on each exit node have to be propagated through the network, and if every exit node did this then it would adversely affect the network in some way that I don't recall the details of.
I don't think bbc.co.uk is blocked in any country.
China, some of the time: http://www.bbc.co.uk/news/world-asia-pacific-11962520
What would be great would be if you could restrict the websites people could go to. I would gladly offer people in internet-oppressed countries access to bbc.co.uk (only).