Hi there!
I'm currently in year 13 just about to finish studying Maths, Further Maths, Physics and Product Design. I also did Geography and Product Design in GCSE and absolutely loved both, so faced the exact same predicament as you do, the only difference being that I didn't know what I wanted to do in UNI! so you're in a better position to choose..
I've firmed Bristol and insurance'd Bath to do Mechanical/General Engineering now.
Bristol gave me a reduced offer because I do P.D as a fourth A-level!
When I was picking A-levels I had to pick between Geo and PD - it was so hard, but I definitely don't regret my decision. PD is very project based and teaches you so many skills you'll find fantastically useful in your Engineering course. For engineering you'll be doing at least a project every year depending on your discipline and PD will prepare you for that!
Time management, presentation, product development, client interface, working with other people, workshop skills.. the list goes on and on.
On the flip side, (im speaking for AQA 3D design product design) I hated the coursework side of the subject, having to slave over it and spend so much time on it, but towards the end i realized it's an important skill to be able to translate the technical facets of your design process and thinking to somebody totally alien to your project - communication is a highly valued 'soft' skill in the workplace. (as is project management and all the others mentioned earlier)
Furthermore P.D is a relatively new subject and GENERALLY isn't 'respected' as much as Physics for example, but for any Engineering course it is recognised.
In my UNI interviews all i talked about is my P.D product and they loved it, if you want a creative outlet then definitely pick P.D. Everybody i know who picked all academically-heavy subjects have regretted not having a subject that's creative and different.
Sorry for the long-winded reply, feel free to ask any more questions!