I know you're looking for CATS info, however to clear up any potential confusion, it is true BGS does a 2 module ATPL theory course with 8 and then 6 exams, but that doesn't mean you have to sit all 8 and then all 6 in 2 sittings.
I did module 1 (8 subjects) brush up, followed by 6 exams one month, 2 the month after. Module 2 I did 5 then 1 the month after.
Worked for me. Even if you chose CATS, once you've studied all the subjects, you'll probably end up using BSG question bank anyway.