Hi mate.
Yeah respect to you for the drive you have! It's brilliant!
Take your time with the grades, if yours still living with the 'rants' then you wont have many outgoings, you can put in the time to get descent grades.. A-C definitely! English maths science included.. (you really really dont use physics at all in the ATPLs, I did my Alevel in it... Although Maths you will need)
And just about the ATPLs... well dont think its all flying based... nice aerofoils and lift equations and learning about instruments and how they work (One of many examples).... I found that interesting..
But believe me, you have to learn A LOT of rubbish, and its a stuggle for people even if you love flying... Lots of people wer disinterested in Human Performance, or Airlaw.
Learning about .. well... just utter rubbish you will never use again to be frank with you... (Out of the 14 books i can think of soooo many examples)
Of course it's nice to know information.... but you never use or think about it in commercial land.
The exams are more... training for the type rating in a way. It's a way for the CAA to see how much information you can take in, how well you can learn it, and how quickly you can get to grips with the pace of learning.... it is so quick!
Just like the type rating (a 2 week course to learn everything about a plane??! Major work.. It's not impossible! Just takes effort).
Good luck matey!
And don't forget, you're not just getting grades to get into aviation as a pilot.
Think of back up plans, you will always need good grades when applying for any job.... even if it maybe a job to help you pay back the training cost.
Good luck!