Flexible school admissions for Summerborns FB group:
www.facebook.com/groups/121613774658942/
Admissions Code 2014, the relevant parts are Section 2, parts 17, 17a and 17b
Advice on the Admission of Summerborn Children:
www.gov.uk/government/uploads/system/uploads/attachment_data/file/389388/School_Admissions_Code_2014_-_19_Dec.pdf
www.gov.uk/government/uploads/system/uploads/attachment_data/file/389448/Summer_born_admissions_advice_Dec_2014.pdf
That's the legal side. In terms of whether you should start them out of cohort, that really is down to you. Some may tell you that your child will be bored or will be bullied when the kids find out that their birthday is a year older than them. My feeling is that a good school will manage bullying effectively regardless of cause and a school that doesn't will have greater problems than just this.
Likewise I don't see why a child born in July will be more bored than a child born two months later in September. I would expect a good school to be able to engage a child regardless of their ability. Also while a child might be academically advanced they could be emotionally immature, and lacking development in other ways. Learning at primary is about more than reading and maths.
What you may find is that because the rules haven't changed down the line YET (this is expected but not in the pipeline as yet) when it comes to secondary school you will have to apply again for your child to start in year 7 and not year 8. At this point, it is up to each school's head teacher so you need to get a yes from every school you put on your application.
It may also affect your child if you are in a grammar school area as the 11+ is age dependant.
However it is not an issue for GCSE or A Levels as these are not age dependent.