I was only having a conversation about this today, how weird is that!
As a Year R teacher, I have to say, please think carefully about deferring for a year. The first year SHOULD be a fun introduction to school, where children play a lot and learn a little, but where they build lots of friendships (providing it is a good year R class). If a child misses the first year, they start the following September as a new child, who has to integrate into existing friendship groups. From an academic point of view, they will have missed a lot of the foundations in terms of phonics and number work, so be prepared for the fact that, to begin with at least, they will be in the bottom groups for most things as they will have some catching up to do. Also in our school, year 1 is a very structured environment e.g. lots of whole class teaching, followed by sitting at desks writing, with very little time to play. In my opinion that would be a huge shock to a child who had experienced nothing of school before.
I think that it is a difficult decision to make and I am glad that Ella isn't a summer baby so I don't have to worry! In my experience 99% of children are ready for school. How does the school manage the start e.g. our summers didn't go full time for ages (not until almost xmas) so had a nice gradual introduction to school (unlike they would get going straight into year 1). If they don't offer this as standard, perhaps you could request it so that Hannah starts in Year R, but slowly.
Sorry if I went on a bit there! I just wanted to put the other side across. Let me know if you want to know anything else.