Looking for a similar solution, but the info i have found to date appears to say although app moved to SD card it still downloads to internal storage. The explanation being it is the app itself that does this as material downloaded is automatically deleted after a given period. It if was downloaded on to an external card, you could remove the card and prevent deletion.
All I suppose we can do is move everything we can from the internal to external card to free as much space as possible for the iplayer to use!
iplayerhelp.external.bbc.co.uk/tv/faqs_mobile_downloads
Quote from the above link:-
Can I download programmes and store them to an SD card?
No. Downloads are only able to download to internal storage. This follows Google developer guidelines for this type of media. Also, due to obligations to our rights holders, a downloaded programme can only be played on the device on which it was downloaded