Not helpful but a weird thing I noticed today - looking at flights from Heathrow to San Francisco on expedia. The exact same flights with a fare difference of over £100 depending upon whether the airline was listed as BMI or United. Weird.
I've noticed that with BMI and more than one of the Star Alliance airlines (my usual pet peeve has been with LH) over the years. The weirdest of all was getting a fare that was about 70 quid less from BMI on a codeshare flight to Munich that was actually operated by LH both ways.
Moral of the story: always look at codeshare partners as well. Sometimes you'll find an amazing bargain.