OK - now I get it.
If you export to Quicktime from iMovie, you get choices of quality, including all the main HD formats.
To create BR discs, try Toast Titanium. I know i means spending money (ouch!), but it'll enable you to create BR discs.
Also, have a look at MPEG-Streamclip, free utility to convert video formats. Very useful and will allow you to spend hours and hours fiddling around (well, that's what I did)
http://www.squared5.com/svideo/mpeg-streamclip-mac.html