BBC PHP framework is Zend Framework, it's sad to see such a lie as one of the main selling points. If Symfony is used at all in BBC is perhaps only to experiment the alternatives.
It's not an national thing but simply because each group has its own requirements and - unless there is strict governance over tools - people choose different solutions for different tradeoffs and things drift.
That history about Silex is ONE developer testing it on a pet project, which is a completely different thing than saying "BBC uses Symfony"
https://speakerdeck.com/u/jcleveley/p/how-we-built-the-new-r...