Most people are in a state school catchment area, but many may luck out if a lot of people move to an area for a particular school.
Pre-prep, IIRC is pretty much equivalent, age wise, to infants in the state sector ie 4-7. Prep is junior - 7-11 - with the emphasis, in the name, on preparing for a selective senior or secondary school. Many private senior or secondary schools and most state secondary schools are non-selective, but you may or may not be in catchment of state grammar schools, which are selective, based on ability (and, many would say, ability to pay for the tuition necessary to excel in various 11+ exams).
Of course, when it comes to assessing which schools would suit your family, google is very much your friend! Your DH might be harder to persuade, when it comes to various options, but you need to decide whether an elite, paid for privately, education is what you want for your child (as other have advised, get planning now, if that's the case) or a more relaxed private education, or a "high flying" state school (choose your house carefully and be prepared for the cost discrepancy to outweigh private school fees, as it even does up here in the Northeast, for some schools) or that, actually, your kids will go to the local bog standard state primary and actually have a bloody great time and learn loads, both in and out of the classroom, regardless of any money or heartache you poured into the situation.
I the end, only you and your stubborn DH, can work out what will be best for your family, as a whole.