Application is through UCAS to social work degree courses. Entry is competitive for some courses
You need A-levels or an access course.
A good way of getting onto a course, and to be paid a reasonable wage to study, is via a trainee social worker post. Basically, councils employ you as a trainee SW, and fund your studies, in return for you working for them on completion of the course. Vacancies are normally advertised on local council websites