Hi Goldcoaster
We had Carshalton High School as our first choice for DD (for entry in Sept. 2017) but unfortunately we didn't get it (live too far away). Previously I think it's been a case that everyone who has wanted a place has got one but the fact that there is now a 'catchment' area would suggest that the school's popularity is going from strength to strength.
We were very impressed on the Open Evening and DD really loved the vibe, so she was sad not to get in, particularly as one of her best friends is going there.
They have recently (just this past year) introduced 'Sports' and 'Music' scholarships for girls particularly able in one/both of those fields. Gives an added positive dimension and is something for local girls to aspire to if they're not necessarily super academic.
It seemed very culturally and socially mixed (which is really what one would want/expect from a comprehensive) from what we saw on the Open Evening, although appreciate that this doesn't necessarily reflect the full picture!
You can really only get a feel for if a school is right for your DD by visiting with her and determining if it's a good fit. Also worth driving/walking around the area at school start/end times - you will get more of an insight of what the girls are like 'out of school' (behaviour, adhering to uniform rules etc...) which often speaks volumes.
If you can visit any of the schools you're interested in this side of the summer hols (most now hold July and September/October Open Day/Evening events), so much the better. It means you have another opportunity to visit in the autumn term if you have shortlisted them on the basis of initial visits.
Good luck!