There has to be a cut off date somewhere. If it was in JUne then people whose birthday fall in July would moan. That's just the way it is.
What I do know is September has a higher percentage of retirements due to to Graduate schemes starting in September, so perhaps that is why. Also it is the start of autumn so the colder nights are drawing in.
Also, and probably the main reason why there is no sliding scale is merely a matter of saving money. We are talking about the same government who have introduced over 35 stealth taxes, tax the dea, increased NHS payments, cut pensions, stopped MIRAS, put petrol up to a world time high. Need I go on?