In my GCSE English exam, I have to describe a place, an event or a person, I think.Would I be able to write a fantastic description to memorise and then modify in the exam to suit the question?
As somebody who has taught GCSE English but also marked GCSE English Language exams, let me give you some advice. What the exam is testing your ability to do is to write attractive and eye-catching descriptions of a certain scenario, event or person. Part of the mark is based on your ability to use figurative language (similes, metaphors etc.) yourself to convey a distinct visual image of the thing you are trying to write about. Because of this, learning an excellent descriptive passage off by heart is not going to help you, as you have no way of predicting what you will be asked to write about and your description may not match the thing you have to describe in the exam.
It is much better to practise at this stage writing descriptive pieces for a wide range of possible topics that you could receive in your exam. How would you describe your earliest childhood memory, for example? How would you describe the person who has had most impact on you in your life? You need to brainstorm these topics, identifying what is notable about them, and then trying to remember what sounds, sights, smells, feelings, and so on that you associate with them. Then you need to consider how you can use words to bring that person, or that event, or that place, to life. I would argue this is the best thing you can do to help prepare yourself for the exam. Learning a descriptive piece by heart will not help you in the exam and it will come across as unnatural and stilted. Good luck!