I think people are misunderstanding the point I was trying to make..
If the choice was between NLCS + CLSG....or HBS and NLCS....or SHHS and CLSG...I wouldn't even mention SPGS (particularly as it divides opinion so strongly...and the mere mention of its name tends to hi-jack threads unintentionally as everyone piles in..)
But...it is undeniable that G&L lives in the "shadow" of SPGS...and is perceived by some as a back-up school in West London.
I'm not sure where OP lives...but in the West London area it appears to be a popular second choice to SPGS...and in many cases 1st choice for girls who don't fancy the perceived arrogance / academic intensity of SPGS.
If you picked up G&L (and its current achievements in University Destinations etc) and plonked it down in another part of London...I'm sure it would attract a lot of local support...and in time develop an even stronger academic reputation than it does now.
i.e. G&L is rather unfairly under the spotlight of "not being SPGS" in a way that many other schools in other parts of London aren't.
As another poster put it, London is importing "East Coast" practices from the US when it comes to schooling...not really that surprising when you think about the influx of overseas buyers of prime London property.
We can all bemoan this...think dreamily about the "good old days"...but current trends suggest it will only intensify over coming years.
The most relaxed families that I know live outside London...attracted by fresh air / more space / + good local schools.
The "arms race" in Central London is being led by "International" families seeking to secure every possible advantage to secure the best education for their kids.
(See the famous "statesmom" thread for jaw-dropping attitudes in this regard)
I thought that I had been "thorough" in exploring the educational opportunities for my daughter... but every interaction I have with an Asian tiger-mom / US wife "tasked" with getting little Joe into Westminster ...makes me realise I have been a complete bumbling amateur.
My prediction would be that in 20-30 years time...it is possible for the academic standards at G&L to be second only to SPGS within London.
It will benefit from Schelling's product clustering effect and the huge excess of demand for school places from affluent well-connected international families living in West / Central London.
Alongside SPGS it will have developed an international reputation for high quality academic teaching...and an expertise for getting girls into top US Universities + other top Universities around the world...as well as Oxbridge etc.
A quota system will eventually need to be developed by schools such as these to maintain their charity status...i.e. they must commit to send a certain proportion to domestic Universities or provide entry for a minimum quota educated within the UK State system...
Anyway...back to present-day:
CLSG benefits from a reputation of being a "destination" school which draws in from a huge catchment area across London.
For many girls...CLSG will be their first choice school...for others it will be their second choice...but the feeling you get is that whatever the historical preferences were...these are forgotten pretty quickly...everyone rolls their sleeves up + gets on with it.
It is exactly the sort of school I would feel very comfortable sending my daughter to from a state primary...without overdue worries about "fitting in" (50 / 50 split at year 7 entry point between State + Independent...not sure off-hand whether similar for G&L)
If I were OP...I would seek out Ena Harrop directly at CLSG open-day next Wednesday and explore her thoughts about IB vs A-Levels / US University admission vs Oxbridge / domestic etc and see what she says.
The good news is that she is a new appointment (so half a chance she will be there for entirety of your DDs education at the school...so her views will be pretty important)
But she comes from a hands-on academic background (current Director of Studies) so does she have marketing drive to put CLSG "on the map" with US Ivy league...in the same way that Bernice McCabe fights her corner for NLCS?
I would ignore some of the criticism you are getting for considering US University admissions...even at this early stage.
If it is quite likely that this will be an option for your daughter... you are being perfectly sensible in exploring the track-record / likely amount of assistance you will be getting from a school in this regard.
Education (for parents) is about keeping as many options open as long as possible...when it is possible to do so.
Many posters having a dig about US Universities...would not send their DC to a particular school if they felt it was clearly harming their chances of achieving a suitable University destination be it Russell Group / Imperial / Oxbridge or whatever.
I'm more North London than West London...so an equivalent choice here would be between SHHS and CLSG.
Again... local "perceived wisdom" is that CLSG is far stronger academically than SHHS...but when you dig a bit deeper into the results...the differences are not that great.
It's very easy to confuse difficulty of access with academic standards of intake...SHHS (and presumably G&L) is easier to get an offer from than CLSG...but that is partly because it is attracting applications on a more local rather than London-wide basis...with relatively fewer girls having it as first choice...so greater need to over-offer places.
In the end we would probably have chosen CLSG...but the decision would have been very close...and had CLSG been another 10 mins away we would have favoured the more local school in SHHS.