Hey!
I've just recently been accepted to the course so I'll try to give you my honest opinion (though it may not be of much use)!
This may sound cheesy but, be yourself. You're obviously hooked on the idea of the course and really, really want to study it, so you're exactly the kind of person that they're looking for. If you want a sweeping statement, they appear to be looking for people who would normally apply for an engineering course but who are more creatively inclined. I guess that's an easy way to put it. But they're definitely looking for somebody who has that scientific background.
I'm taking Maths, Physics and Music to Scottish Advanced Higher level if that helps too.
Oh yes! One other tip, don't tailor your personal statement just towards Design Engineering unless all the other courses that you're taking are similar to Product Design Engineering etc. My personal statement was written entirely for an application to Electrical and Electronic Engineering and I ended up emailing Imperial's admission board asking for my application to be transferred across to Design Engineering rather than EEE. So yeah, write a statement for what the majority of your courses are, but try to work in some little things that show off your love for design, innovation and creativity.
I hope this helps! And feel free to PM with any more questions as you think of them.