They wont be marked out of 100, eg. maths ext 1 is out of 84, math and maths ext 2 is out of 120. However they are reported to you as out of 50 or 100
4 unit will be reported out of 100.
A 1 unit subject such as studies of religion or if you only do maths/english ext 1 without ext 2 will be reported out 50
I'd say topping bio is better than top half of physics, but it depends on the school and how smart your physics class is and which one you are better in