There is no mixed prep school in the area which is why I spend half my life driving. The closest thing is Surbiton High which also runs a boys prep. My dd is in SH for the guaranteed 11+ place, but her brother is elsewhere in a 13+ school as are many siblings. I saw the results for Surbiton Prep and they did very well and got a lot of offers at very good schools, and a lot of Tiffin offers, but we were only interested in 13+ schools so never considered it for my ds. Off the top of my head Surbiton and Denmead are the only local boys schools I can think of who will prepare for 11+ as they both finish at year 6.
The nearest mixed prep is probably Newland House in Teddington. They are loosing an awful lot of Y6 boys this year as well as all the girls, its always been well thought of, they have a new head who people speak well of, and I really don't know why so many are leaving. Twickenham Prep is mixed and driveable from Kingston, again a very good mixed prep. Weston Green were looking to extend their pre-prep to year 6, but it would be a very small school and I am not sure how many extra years they have taken on so far. Milbourne Lodge is also co-edish, if you look long enough and hard enough you may spot a girl, but its a 13+ school and won't prepare for 11+ exams.
St Georges runs buses out to Kingston, and would have to prepare pupils for their own 11+ exam.
There are lots of very good single sex school preps in the area, but boys schools finish at 13 and will not prepare for the 11+. At least at a mixed school they have to prepare the girls and the boys are usually included.
Good Luck looking.