The UK emphasise on concentrating in one area of academic pursuit. Admissions Offices want to see that you are genuinely curious about what you are going to study.
You prepare one essay, this is sent to all the schools you are applying to.
The structure of UCAS Essays normally follows:
1. One paragraph introduction of how your personal experience prompted your interest in the field of studies
2. Two to three paragraphs on books, research projects, summer courses, and class content that you’ve engaged with which further inspired your studies. Mention how you’d like to extend beyond your knowledge in other particular fields.
3. One paragraph briefly mentioning other non-academic extracurriculars you’ve pursued in (not super necessary if you are short on character count, but worth mentioning if you’ve performed particularly well in them)
It’s very important to use formal language and convey your intellectual curiosity. The main things admissions officers will look at is point 2 in the list above. This means preparing for these experiences in younger years is very important.