My DS2 was born 3 weeks early and so is August instead of September and is the youngest and smallest in his class.
He is small for his age anyway and my DH really wanted him to repeat a year so he would be the oldest and more average in size.
Academically, however, he is in the top sets for everything (going into yr9 next week) He has also one of the very sporty boys and good at all sports except rugby which he loathes (due to his size??)
(this is NOT stealth boasting)
But my DH thinks he could feel more confident within himself if he was the oldest and of more average height in his year group. Certainly, despite the fact that he is a high achiever my DS2 has low confidence and was bullied last year by a clique of boys in his class.
We have even discussed moving to Australia or France where the children start a year later anyway and in Australia the years run from January I think so he would be in the middle age-wise.
This concern of my DH stems from a book he read, Outliers by Malcom Gladwell, its all about how significant your birthday is in terms of how successful you are as a sportsman.
If you read this you will definitely want to move your DS down a year!
however, from my DH's investigations into this, you will not be able to do this here in England, regardless of whether he goes to state or independent.