The CAA does not need any British formal education requirements for a student to commence an ATPL course. Any entrance requirements you may come across will be those of the individual school.
Taking the FTO at which I work as an example, we normally expect students on entry to us to have GCSE English and Maths. This is the standard of exam that 16-year-olds sit in the UK. However, if some student from another country wishes to join our course, as long as they can satisfy us that they have gained an equivalent standard in their own country, we are happy to accept them. I am sure that most other schools would operate with similar rules, but you would have to check with the school of your choice.