Take the time to thoroughly prepare for any admissions exam you have to take. They are the main gatekeepers of admissions decisions.
If you don’t hit the range a university is looking for, it is unlikely the rest of your application will be considered. American students applying to graduate schools will have been familiar with these tests for several years before they take them.
However, British students are capable of performing excellently and achieving the highest possible scores.
You should sit several full, timed practice tests including the writing portions, and plan a revision programme focussing on areas you need to improve.