So this might not be helpful but seeing as you are quite keen on heading south I will go into more detail about our further out schools in case it helps.
If you went further south to Redhill area you would have a four preps to choose from: Reigate St Marys, Chinthurst, Micklefield or Hawthorns.
I would expect there to be a number of families of Asian descent at all the schools, but it might be the case that your child would be the only one in their class.
The pluses are that they are all co-ed with high academic standards. Also gives you options to move the kids if a particular school ends up not being the perfect fit. This does happen, because you can't always tell your childs strengths before they start.
RSM is a feeder to Reigate Grammar. Pro to this is 80 per cent of the kids get offered a place without having to sit the 11 plus. Con is that it can be a difficult place to be for a non academic child/child that doesn't make the grade in year 5 for RGS. Inevitably prep for other destinations is not as strong. Chinthurst is also part of this deal but a smaller school with a somewhat wider spread of school destinations.
Hawthorns runs until 13, so a back up if 11 plus process is difficult. Lovely setting and facilities. This is the school I personally know the least. Friend who has a child that goes there fit into RGS last year without tutoring.
Micklefield is more inclusive and smaller. My kids are there they and I love it, the headmaster is great, the school supportive but also challenging. It has definitely surprised me in a positive way. Kids prepped for a variety of destinations, but RGS, Caterham and Dunottar are popular. I have no intention of tutoring (and what is nearing that age) - if child can't get in with the extra prep/support the school provides I don't think it is the right place for them.
For me, the big pro of this area is the secondary options. RGS for very academic children and dunottar for the more sporty/musical. Both day schools. I am not keen on my children ending up as day pupils at boarding schools so Sevenoaks (where my sibling went) wouldn't be for us. We also prefer co-ed for secondary. If you prefer single sex for secondary then Sutton/Croydon area is better.