I don't do physics, comp sci but i do maths and further maths
For maths definitely do Edexcel - they have a very easy to use textbook series by pearson and have most resources around in general (e.g. on YouTube). Also grade boundaries are low (60% for A).
Also use Seneca for studying for all topics, for any course and its free.
CAIE is very hard and high grade boundaries, because a lot of private schools/ international schools use this course