You don't need French for uni, you've established that, so it's really up to you whether you really want to put the work in to get a higher mark. I would have thought that your A2 year should be about putting effort into the subjects that you need for uni, which will be Art, Fashion and Maths. You're already retaking Maths so unless you really think you can cope with retaking a few more French modules as well, I would just leave it.
I would say don't take French on to A2 - as has already been said, it will be harder at A2 and you'll need a lot of work to get a higher grade. I'm sorry you didn't get what you wanted, but if you got better in your mock, that's a good case for a resit or two (be wary of doing too many: you don't want to jepodise your other subjects) with a private tutor. An E in an unrelated subject really doesn't matter that much at all, but if you can improve on it, then great.
Carry on your best three - that's always the most common thing to do, and for good reasons. If you want to, re-take C1 and/or C2 next summer, as C3 and C4 build so much on the AS pure stuff that they'll seem so easy and you'll pick up the extra marks to give you a bit of a cushion for an A next year.
Well done for your A in art by the way - from my friend doing art A-level, I know there's a fantastic amount of work you need to put in, as well as having to be very talented in the first place! I'm very impressed (and jealous!).