You clearly need to ensure that your maths and physics are a minimum standard prior to undertaking ATPL studies.
As you intend to take the modular route, you need to obtain your PPL first.
The flying club you choose will be able to give you appropriate guidance.
Also the ground school provider will be ideally placed to advise you on pre entry studies.
As there is a time restriction / number of attempts allowed on passing the exam, you need to concentrate 100% on them.
Do the OU if you wish either before, or after, but not during the ATPL studies/exams.
So far as the airlines as concerned, as a modular student, is how well you progress during the training. I commented on this in the previous post.