I?ve re-read your OP right from the beginning, oxomum and have concluded that you are having too many misconceptions and wrongly assuming too many things as fact.
(1) Yes, Dragon?s 50% scholarship looks interesting but don?t be surprised if they also send you a Bursary Form of sorts (and I think they will) and ask you to complete (assuming your son passed all the tests, etc.).
(2) Although they don?t usually say it, most top prep schools have no problems in giving 50% or even 100% scholarships to one or two children who are exceptional in their eyes i.e. above Level 5 at age 8. It's up to you to ask and to draw their attention to your child.
(3) With a family income of £70K p.a. does not necessarily mean one can afford to send one?s child to a private school. They don't know what are your other financial committments. Without meaning to be personal or patronising, you are a good case in point, otherwise you won?t be looking at the 50% Dragon scholarship so closely.
(4) For example, if you look at Eton?s Bursary Form closely, they even ask what you have in terms of number of properties, their acreage(s), collections, paintings, works of art, vintage cars, farms, silver 
etc . The inference here being there are people who own these assets and still need a bursary!
(5) Beside princes and lords who live in castles and Britain?s most expensive post codes, there are Etonians whose parents are unemployed, live in tough council estates and work as general cleaners. By wearing the same uniform, your son (if he goes there) probably won?t know who is a prince and who is a pauper, and neither do they because nobody ever asked. He therefore, would not feel out of place even in years to come when he?s actually talking to a real Prince or President in his tailcoat at an official function. Others without this experience would probably feel like a penguin gone astray form the Antarctica.