Start by installing Homebrew and use this to install ffmpeg:

/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
brew link python
brew install ffmpeg
brew install ffmpeg --with-fdk-aac --with-ffplay --with-freetype --with-frei0r --with-libass --with-libvo-aacenc --with-libvorbis --with-libvpx --with-opencore-amr --with-openjpeg --with-opus --with-rtmpdump --with-schroedinger --with-speex --with-theora --with-tools

Now install the TamperMonkey plugin for your Chrome browser and install the questo script https://greasyfork.org/it/scripts/8161-bbc-iplayer-video-download

If you open a video on iplayer you will see a script under the video. Run this using Terminal to download the video.

http://www.renevolution.com/ffmpeg/2013/03/16/how-to-install-ffmpeg-on-mac-os-x.html

https://andrealazzarotto.com/2015/02/20/how-to-download-bbc-iplayer-videos/