Q: download in itunes tv @ sd quality

Hi,

I've got nearly a terabyte of tv and movies purchased  in iTunes. However when I try to select a lower quality for download I cannot switch to it successfully.

iv only got a 128gb iPad so I want to maximise what I put on there for travelling as I can be away from internet for weeks at a time, and sd versions can double the content I can view

I was also looking at buying an iMac but with standard 500gb HDD I again would have to be judicious about downloads.

I've searched other forums and this seems to be an issue in other countries, ive tried using iTunes on my pc and the iTunes app on my ipad and iPhone to no avail.