They use it in multiple languages (World Service), mediums (TV, online, print), and use-cases (serious for news, legible for preschoolers learning to read, edgy for comedy trailers, etc.), for which the web fonts may be too limited or restrictive in number and variation of styles or characters.

It's also an opportunity to brand something as uniquely theirs, unlike a Google web font that anybody can use.

By curiosity, are they guaranteed to be free forever ?

I know that the current licensing is as open as it can be, but if Google or the actual legal owner changes its mind, will the current users need to go through a clearing of everything they have that uses the font ?

I would guess there’s no guarantee, and wouldn’t take that risk if I was in the BBC.