Unless you are on an Integrated course you cannot start your ATPL theoretical knowledge course until you have your PPL IN YOUR HAND.
The reason that I have stressed "IN YOUR HAND" is because when the CAA are in one of their (quite frequent) difficult moods, they will insist that it is not enough to have passed all of the PPL exams and flight tests. You must have actually had your PPL issued.
You can of course start reading up on ATPL material before you get your PPL. But it would probably be more productive to sharpen up on maths and revise your PPL theory. Most students starting ATPL have passed their PPL simply by learning answers from the PPL Confuser. This means that they have none of the basic knowledge that they should have at the start of the ATPL. This does not prevent them from taking in the ATPL material but it certainly makes it more difficult.