You've had lots of good advice already about contacting the LA and the schools you are looking to apply to for their policy and stance on it. I definitely wouldn't go for it if any of them make a deferred child go straight into Year 1 as the Reception year is so important to help them settle into the school while still following the same Early Years curriculum they are already following at nursery.
If they do let them completely defer into the next year's cohort it is something to consider but you'll still need to apply and them state your decision to defer before she is due to start.
It's definitely not a decision I'd take lightly. The Reception year is like an extension of nursery and, as an experienced Reception teacher, believe me when I say that many Summer borns do cope absolutely fine. They mature and change so much in such a short period of time at this age. I wouldn't expect any 3 year old August born child to seem ready for school yet, so I completely understand your fears, but many with September/October born children who are already 4 often feel their child would have been ready to go this year and worry about them being bored of nursery by Easter and ready for more stimulation. So by the summer you may have noticed a big change in your daughter and see things differently.
Some children would benefit massively from an extra year of nursery, others will end up bored by Christmas if you hold them back based on age only.
So find out when you need to defer by, if indeed you can, and make the decision as late as possible, based on your child's individual needs, rather than just her date of birth.