The course at Leiden is taught in English. So, the chances of getting onto the 4-year course look slim. And you should not think that, just because there are few places on the course, it would be a waste of one of your UCAS choices. Candidates are interviewed in December, and sit a one-hour written test while they are here for interview. It is not necessary for candidates to make any particular preparation for the written test.
Most successful candidates go on to get three or four A grades at A level.
Frankly, it does not much matter. Most candidates have done no Law at school, although occasionally we have an applicant who is doing Law GCSE or A level. Many candidates' A-level subjects will include History, French, English or Maths, but we are happy to receive applications from candidates taking any combination of subjects. And we often have successful candidates who are taking full science A levels. We interview as many candidates as possible.