I did both Chemistry and French A level. Dropped French after AS (this was all a good 4 years ago so the exams have obviously changed since then).
I found gcse French a walk in the park compared to a level. That was mainly down to the writing and oral exam. At gcse we knew exactly what the writing question was and we would learn our reply. The teacher would proof read it several times then we would just learn the perfect version. The same went for the speaking exam, we knew what questions we would be asked and we learn the perfect answers so getting 100% in those exams was a piece of cake.
A level it was more spontaneous because you hadn't a clue what exact questions you would get. You hadn't to just be really good at improvising and conversing in French for the speaking then the writing was just something else! Such a step up from gcse. The rest of it i.e. Listening and reading exams were a little more difficult too. A2 French I didn't do but it was literature based so you would read a French novel and talk about it. Really hard, I got a D at AS, had done French since the age of 4 and I'm barely able to hold a conversation let alone be fluent which is what I hoped to achieve by doing the A level.
I would say don't do a level French unless you are naturally good at it and need it for your future. If, like me, you just want to speak the language then just spend 6 months in the damn country!
Chemistry was my best science at GSCE. I did biology A level as well (and drama, I know what a mix) and I got B's in both the sciences, chemistry being the slightly better one. Overall chemistry was fine, I had really good teachers. As far as the maths side goes it was pretty minimal to be fair and you were allowed a calculator. There was just a lot of equations and different things to learn.
I would say do chemistry but that's just me. I think it will be moreover valuable especially considering the other subjects you plan on studying.
I was in a similar situation in terms of not focusing in lessons with chemistry but a level chemistry. The first unit which was the easiest I barely took in anything during the lessons, did mediocrely in the end of unit tests in class then got a B in the exam at AS. Re took that unit the following year and got near full UMS because I realized it was actually pretty easy. That was at the expense of the other A2 units I also had to take plus ISAs (which were easy actually).
Chemistry was more fun, loved the practical side of things. You will make the right decision for you though. At the end of the day, do the subject you enjoy most!