Drawing and experiments are what really gives you that kind of good marks.
Focus on the minimal layout of your sketchbook, and focus your energy what will actually be graded, your work and sketches!
Your current level of skill does not really impact your grade as it is not an art exam. All of the final pieces can be pre-traced, and during the exam you will just be good at shading.
My advice to you experiment with different mediums in one art project. Try to use charcoal, pencils, biro pens, acrylic paint, and loads more. Get out of your comfort zone, really focus on trying new things
Show all of your sketches which are rough, do mind maps and just focus on the exploration of concepts. Examiners really value if you show them the step by step how you get from point A to point Z. Your art skills do not really matter at this time.
If topic is not inspiring, then it will be very hard to actually do anything about it. Make sure you have fun with your topic and sketchbook.
Try something that you have not yet tried and explore your curiosities!
When you try new things, experiment with different art supplies then you can for sure come up with something unique.
That’s all! Honestly, just enjoy the process, try new things and everything will be great! Good luck with your GCSE Art.