To be exempted from sitting the 14 CAA ATPL exams you need more than 3000 hours on aircraft > 30,000kgs. Even then you still need to pass the Human Performance and Air Law exams.
Otherwise if you have at least 1500 hours on two-pilot aircraft you need to pass all 14 ATPL exams, but are exempted from formal groundcourse component.
If you don't fit into any of the above you need to enrol in a formal groundschool by either distance or full time, and pass the 14 exams.
Cheers.