Netflix has posted the strangely absent up-till-now Season 6 ("Volume 6") of MI-5, making all 10 Seasons now available. They seem to be in order and only Season 1, Episode 2 is missing and "disc only." Fortunately, Idaho Public Television has it posted here, and since it's on their site I can only assume that it is a legal copy: http://video.idahoptv.org/video/2047610596/
With that, you can now enjoy all 10 seasons of the show. Let us know if the video isn't available in your region or if anything seems out of order in the comments.