Blinkbox has made a deal with WB so that you can "download and own" Babylon 5 episodes, although I get the "not available in your country message." (This one is a UK site, so I'm sure it's the same deal with thewb.com not working in the UK since that site is in the US). Also, only season 1 episodes are up, and for some reason "The Quality of Mercy" is not there.
http://www.blinkbox.com/TV/Series/290/Babylon-5
http://www.blinkbox.com/TV/Series/290/Babylon-5