They are not actively biased, they just take the top results. One could argue that a coed school should accept a level of up to 2 years lower in boys because they develop later, but I'm sure most would deem that unfair or even ridiculous. Not every August birthday is at a disadvantage. I expect when children are assessed
at 4 years old in London schools then birth date may be taken into consideration. By 11, when they are formally examined, it is assumed that most will have caught up. I have friends who did feel their childREN were clearly much younger during the reception year but by Year 3/4 there was no difference.