Yes, you're right; generally, students tend to stick with the same friends from secondary school. However, if your child is in the top sets and performs well, many Sixth Forms will gladly accept him. Additionally, some girls' schools, like Waldegrave, accept boys for A-levels.
It's also worth remembering that your child's A-levels are still several years away, and many things could change by then. For instance, around ten years ago, GC School had poor results and behaviour, but with a new headteacher and improvements like the Inner Circle catchment area for Tiffin, the situation has changed significantly. Now, families often buy homes in the Inner Circle area, and if their child doesn’t get into Tiffin, they choose GC instead.
Years ago, Teddington's results were better than GC's. GC’s performance has since improved, while Teddington went through a rough patch but is now on the rise again.
My son’s school also had similar challenges. It was underperforming, but for the last few years with a new headmaster, it’s now ranked among the top 100 state schools in terms of results.
The conclusion is ...a lot can unexpectedly change over a course of years
About the housing. The better the school the more expensive houses around. I know a person who decided on renting to get her daughter to Waldegrave.