I agree, it feels far too young. However it's worth noting that if you choose to defer, they will likely be put straight into year 1 when they start and will miss their foundation year (reception). My mum is a reception teacher and she advised that the reception year is a really important year for them in terms of school readiness. The jump from reception to year 1 is quite a big one so she advised against deferral and said to consider homeschooling instead if the age is a concern. She has also said not to be too worried about them being the youngest in their year group as she said there's very little difference at that age and some of her brightest children are the youngest in the year, so that's good to know!
Private school does offer a far better standard of education. If you can afford it I don’t think there is any question as to why you wouldn’t want your child to go. Smaller classes, focus on more advanced life skills, more advanced curriculum. We have already registered our son to start September 26 the private school we have chosen as part of their standard curriculum (different to state schools curriculum) all students are taught an instrument of our choosing; piano, drums violin ect (not just the useless recorder) two languages again of our choosing from a list (this is NOT) standard for uk state schools. Whilst viewing the school on an open day the year 6 class was learning about interest rates and mortgages (non standard uk curriculum but invaluable life skills we all need!) we will be starting him at school in September 26. We personally don’t want him to miss reception year and he is already in this schools system so will naturally just move up to pre school then reception.
I personally won't be exploring the private school option as I don't think it's needed. I attended primary school here and feel that as long as it's the right one then it's a good experience (mine was). I think choosing to defer for me is based on your child's ability level and whether you feel they would be ready. For example I know that my daughter will be but if I wasn't sure I would probably defer.