[manarth]

The message they display says:

  We're sorry but this site is not accessible from the UK as it is part of our
  international service and is not funded by the licence fee. It is run commercially
  by BBC Worldwide, a wholly-owned subsidiary of the BBC, the profits
  made from it go back to BBC programme-makers to help fund great
  new BBC programmes.

The rationale is explained in a little more detail in a BBC FAQ [1].

BBC Worldwide is a commercial entity, but the BBC's fair trading rules prevents them from promoting a commercial entity to UK licence payers. The somewhat odd outcome is that BBC content is unavailable to people in the UK.

[1] http://www.bbc.co.uk/faqs/online/website_changes