Britbox is supposed to be that, I think, but I wouldn't be at all surprised if the BBC cannot legally sell iPlayer subscriptions as easily as you'd think because the BBC's copyright library (especially music) is unbelievable[1] compared to most other organizations, so they'd have to make multiple versions of everything on their books to deal with licencing issues. Lots of British shows get their soundtracks neutered because of this when they are sold abroad.

[1] Example: Sky Sports F1 is pretty big budget, but they're still stuck using [knock-off version of The Chain] rather than the real thing, whereas the BBC can break out a really expensive song (e.g. Money for nothing by Dire Straits) for a throwaway segment.