English language (and maths) is a GCSE you need for progression, so if you don't pass, most colleges/6th forms will enter you for a retake and you will take lessons alongside A levels/a BTEC. If you do pass, you would likely have to enter yourself as a private candidate and do it on the side if you want to improve your grade, but it does cost money.
Unis don't tend to care about GCSEs, they're more interested in your A levels. There are some exceptions for courses/unis that require an early application (or are competitive) such as oxbridge/medicine/veterinary courses. Check the specific entry requirements for unis/courses you think you may be interested in, but generally they just require a pass (4/5). A high grade in an essay-based subject would probably be a more accurate reflection of your writing ability nearer the time of attending uni.