If you follow the link given by arkady, you will see that music GCSE is a lot more than playing an instrument. You have to know theory, history, and be able to compose. Performance is 40% of the total mark. This is made up of half each of an individual performance and a group performance.
Some years ago it was possible to put forward your own grade 5 in an instrument as part of the assessment, but that is no longer possible.
The specification says that 3 marks (out of the 30 given for performance) are awarded for complexity of performance if that is equivalent to grade 5, 2 marks for grade 4, and 1 mark for grade 2 or 3.
So if your playing is not very good, you will lose some marks. What is interesting is that your performance can also be technology based.
My advice is to speak to a music teacher at the school, find out what syllabus they are doing and ask about the performance element and what your chances are, based on what you can already do.