It may also be the case that some or all of their content is licensed for a non-UK audience, meaning that they would have higher costs if they wanted to allow UK viewers, a cost which they might not recoup from UK advertisement viewers.

That said, based on the limited information we have, I'm totally with you that it's a ridiculous situation. Maybe they have a somewhat valid reason, such as the one I suggested above, but I do doubt it.

Not exactly - the BBC often serves the same content without displaying ads in the UK.

I'm not clear what's going on in this case, but it's more likely that this is out of its public remit - as a funded public service, they're not to compete with commercial companies in sensitive industries/they're not allowed to use public money on content irrelevant to their remit and so BBC Worldwide has to be somewhat carefully separated.

e.g. http://news.bbc.co.uk/1/hi/entertainment/4058031.stm

This may be down to Rupert Murdoch's lobbying pressure.