You still don't seem understand what CBBC is and what it represents in a UK context - no current prime-time star is going to see moving to star in a show on CBBC aimed at 7-12 year olds as anything but a step backwards or downwards - it a place where careers start and careers wind-down*. Jenna's previous role before Doctor Who was bigger than that...
* outside guest appearances in things like Horrible histories, voiceovers and other things like that.
Yes outside of a bit part in the first Captain America movie her last part was huge, I'd even go so far as to call it titanic.
