The BBC is not allowed to use the TVL money outside of UK content (although they are now having to use it to fund World Service Radio as the government decided to save some money in the foreign office budget).
They are also not allowed to freely use content produced by their foreign broadcast operations in the UK.
In both directions the BBC has to pay/receive market rate for the content to avoid competition issues.
It's also why the BBC have BBC:Worldwide as it's commercial arm, which makes quite a low headline profit, but returns a lot to the BBC - as it has to keep the profit from it's operations separate from it's costs, of which one of the biggest is paying the BBC market rate for content*.
What the BBC does tend to do for some stuff is to share the cost between both the UK TVL funded operation and Worldwide as what would normally be classed as "co-productions", I think things like BBC Click on the news channel is one such program, and I'm fairly sure several drama programmes have both the BBC and BBC Worldwide credits.
IIRC there are parts of the BBC website that you cannot access in the UK because they are actually under the BBC Worldwide arm, and thus paid for commercially.
It's one of the reasons BBC America will show content from all providers, as it buys it in
*Apparently both the BBC and BBCWW have fairly large teams of buyers/sellers whose job it is to buy or sell content to the best bidder, which leads to the rather funny situation where effectively one part of the BBC is basically employing people to get a better deal out of another part of it (a little like some councils might have certain in house services that have to bid for tasks inside the council, with outside contractors also bidding).
