Agenda? What Agenda?
It always bugs me that certain "fans" accuse the BBC of doing this to further some kind of "lefty PC agenda", while being completely ignorant of how casting in done for TV shows. The Beeb don't hover over Doctor Who like some nebulous Big Brother, pointing at Chibnall and intoning "Thou Shalt Cast a Female Doctor for Lo it is the next stage in the great LGBT Master Plan".
Chibnall has total control of who he auditions and who he casts. Yes he's admitted he's always wanted a female Doctor, because he like strong female roles, and wants to see what JW will do with it.
Oh and the other complaint is that he's picked her because she was in his show Broadchurch. Well, duh, yeah why not? He'll contact the agents of actors whose work he is familiar with and ask them to come in and audition. Same reason why RTD auditioned and cast Tennant, who he'd worked with on "Casanova". If Whittaker hadn't pleased him in the audition he wouldn't have cast her, simple as that. But apparently she aced it, so she's the 13th Doctor.
Sometimes (probably most times) there isn't a hidden agenda. There isn't a plot. Sometimes actors get the job because they're damn good at it.
Personally I've never look forward to a new series of Doctor Who this much since 2005.