You describe UK “coursework” as intense and grading as based on “one paper or project” but this might be underestimating the exam-based structure.
UK students are used to a single end of year exam that covers all the material in that subject for the whole year. They will prepare (“cram”) for multiple weeks as self-study to learn all the material for that exam (sitting in your room or the library on your own for 8+ hours a day revising is typical). And exams often require applying known techniques/information to novel problems/questions under time pressure.
Also note that scores are low (70% is a first, which only a third of the class achieves) so you aren’t expect to solve all the questions. And your final degree class is important for job prospects, if you get a first you will put that on your resume for your whole career.
AP tests are the closest experience in the US (hence why they are valued for UK admissions), but the amount of material you have to learn and regurgitate is generally much greater. So you do need to enjoy that style of assessment.