In England children start school the September after their 4th Birthday, in Scotland children start aged 4 but I was under the impression in both cases school isn't compulsory until the child turns 5.
Personally as a Mum with an older child in Year 1 & a teacher I think there is a huge difference in children born in September vs the summer-think about it as babies, my daughter was about to turn 1 when some of her class weren't even born yet! I wouldn't be timing the conception around this though!