I know it's an old thread, but just reading some of your comments, I think although Bromptons do fold up the smallest of all the folders, they're still surprisingly big when you try and get them into a car. I'm guessing more so with the hard case.
We have a Q5 and while the boot isn't huge - certainly not compared to our old A6 - the Brompton effectively takes up more than half of it. It won't go in stood up, so it has to lie down and without any other form of cover, means that you can't put anything on top.