This thread has got me all confused. I don't come from the UK so not sure how the system there work but I am an "August baby" and definitely wasn't the youngest in my class. The youngest ones were the ones born in December where I am from but everyone starts school around the age of 2, 5 or 3 yo. Never noticed that the kids born in December were at a disadvantage, I definitely didn't feel at a disadvantage at the time either and the only slight con of being born in August I noticed was my birthday falling right in the middle of the summer holidays making it hard to have a proper birthday party! (which always suited me perfectly anyway as I always very much disliked being the center of attention).
I had good grades, was very good at sports and picked up languages very quickly.
Maybe December babies did feel at a disadvantage for being younger but I can't say I have noticed a pattern where younger children had worse grades. If anything, my brother is a "February baby" (so was technically among the oldest of his class) and definitely isn't the brightest light around, and I have noticed the same with a few other February kids so I think it's more a personality thing than anything else really.
It's the first time I hear about people willingly trying to avoid their child being born a certain month of the year (25th of December ok, but August?) Mind you, I am quite glad that this isn't really a thing where I am from as, had my mom thought like this, I wouldn't have been born! 
I think there are worse things in life than having a kid that's a bit behind at school or who get slightly lower grades than his peers (regardless of date of birth). You could be granted with a September baby that happen to not be healthy enough to ever attend school, being born a certain month doesn't guarantee the perfect baby (I understand your concern is for your future child not to have to face unnecessary challenges but really even if you plan the perfect pregnancy from A to Z so there is still no way to know what your child will be like so don't let statistic scare you! )
Out of the many things that have affected my life being born in August wasn't one of them. If anything competing with older kids has forced me to always be on top of my game and not rely on my age like other kids liked to do. There isn't really anything challenging in competing with a bunch of 4 yo if you are 5 tbh so I am quite glad i wasn't part of the older ones.