I am now going on to do Engineering/Maths at uni, not 100% sure which yet. I did Art GCSE, as at the time I wanted to be an Architect. I guess this shows how your mind can change.
One thing I found with art, and you will also find, is that if you want to do well (I got an A*) it takes heaps of work - loads. You have to fill sketchbooks, do final pieces, and this is all so much (unneeded stress) alongside your other subjects. If you want to do engineering, I'd say do Computer Science, and focus on getting A*s in Bio Chem and Physics and Maths, and then do Computer Science alongside that, as it will look better than art for engineering.
This is unless of course, you think you may change your mind and apply to an Architecture course when the time comes - for this, you will need Art GCSE and more than likely A-level.
Good luck!