Which is easier singing or playing an instrument decently? Watch
I play instruments and sing. For me singing is normally easier (I can sing grade 8 pieces and play grade 8 pieces on guitar, but it's less time consuming to learn a piece for vocals), but I just can sing. Singing is normally something you either can or can't do in my experience.
Maybe sing a piece for your teacher and see what they think. It's also worth keeping in mind that you only have to play two pieces for your GCSE performance, and they only have to be grade 3 standard. I reckon you could start basically any instrument now and as long as you were dedicated and didn't keep changing your mind about what pieces you want to do, you would be ready by the end of the two years.
I would advise doing what you want to do. If you think you'll enjoy playing an instrument, it's worth doing, even if you can sing as well. Then you'll just be able to do more things, which is good.