I think it's really important to have some context for this. It's not just a matter of "we can't show ads" - more that the BBC cannot enter certain markets in the UK that fall outside their remit.

A number of years ago, the BBC ran a lot of websites that were extremely useful but not directly related to their broadcast output.

Unfortunately (for the general public), commercial broadcasters and publishers made persistant and vocal complaints that they were being irreparably harmed by the BBC entering into markets that were well outside their broadcasting remit. This led to a number of those sites getting shut down, and the BBC's web remit being considerably tightened inside the UK.

BBC Worldwide operates to a certain degree 'at arms length' from the publicly funded BBC, and is able to enter markets that are outside the BBC's normal remit. However, it is severely restrained on entering those markets in the UK, due to the perceived advantage the company has over other commercial competitors.