Read up on GCSE medium level maths and physics.
Maths areas to cover are:
Trigonometry.
Sine, Cosine, Tan on buttons of calculator.
Algebra.
Physics areas to cover are:
Vectors.
Moments.
Tempreture, pressure, density relationship.
A good book to read, although totally above ATPL requirements, is the AC Kermode book 'Mechanics of Flight'. That should help with the Principles of Flight exam. Do not panic if it seems difficult. If you understand all areas of that book you'll be the first!
If you plan to study with Bristol then I shouldn't worry too much as the course notes assume little prior knowledge of maths and science. If you call Bristol I believe they supply a maths/physics refresher pack for potential ATPL students/customers.
Don't worry, I never passed GCSE maths or physics but still passed ATPL's with Bristol
VFE.