We are just ordinary middle class people too but we had our children late, consequently we are able to afford more I feel. We both work full time in good roles, of course there are the super-rich, however there are at least 50% of women working in some capacity as well as the wives of investment bankers at Caldicott. You will be absolutely fine. However, one word of caution. The senior boarding schools will suck you in in a nice way I think, but everyone will talk about Charterhouse, Harrow, Eton and you will find yourself thinking about them as well!
Merchants Taylors I believe is getting very academic and what would happen if your son didnt make it through the test. Would the fallback be a boarding school, would you want to pay all that money, would your son be suitable for a boarding school? Around South Bucks we dont have any day private senior schools apart from Merchants Taylor so you might find yourself in a catch 22, do you move house to increase your choice of day schools (hughly expensive with stamp duty etc) or do you go for a boarding school which will give you lots of choice in the South East.
Also, a parent wanted to move their son from Caldicott to MT's a couple of years ago at 11. There is an unwritten rule that MT dont poach boys from another fee paying school and suggested 13 might be the best age to start. Not impossible of course but just so that you are aware.
Additionally when your son has reached the senior part of Caldicott and starts boarding with all his friends at 11 do you really want to take that option away from him. Boarding is done very gently, and in a safe environment, boys are allowed to be homesick and to not be afraid to cry or show some emotion. My older son who has just started at senior boarding is streets adhead in terms of confidence and attitude and Caldicott taught him that.
When you have the tour dont be nervous about asking ANY questions and be prepared for a very charming Head...