I am currently in Year 12 studying Maths, Further Maths and the new A levels of Computer Science and Physics. My experience of Computer Science A Level isn't the best, but that's due to the poor teaching and the department in my school and not because of the course itself. Computer Science is a lot more interesting in terms of content than the pervious Computing or ICT courses. If you want an idea of what the course entails, look up 'craigndave' on YouTube who have made videos of the whole new AS course which might give you a good idea.
I don't think you have to be a coding genius to do the course. The only real programming you have to do is the programming project in A2 which is 20% of the grade and most of the marks for that if for the report of the project, not nescessarily the code itself. You do need to be quite good at understanding and writing algorithms in the second paper. There are some model papers of this on the OCR website. If you do D1 in Maths, then it covers most of the algorithms you need to know by heart for the Computer Science course.
I would definitely recommend that if you want to do something STEM related you do Further Maths if you can. I do think though with Computer Science is that it has a bit of a stigma about it being esay and pointless. Although you don't need Computer Science A Level to do it at university, the course has changed a lot and the focus is more computational thinking based so you have to take advice from others with a pinch of salt. I really hope this helps!