The BBC won't provide all radio programmes as downloads until they can apply DRM to copyright material. At the moment their 'podcasts' are unprotected mp3. They are implementing new delivery systems - they call it 'audio factory' - to replace the hotch-potch of systems built up over the years and that will be capable of delivering DRM'd audio (HLS/AAC) to their iPlayer apps on various devices including mobile.

This may happen this year, but I've not seen a date. It's been delayed before.