You asked about Tormead. Worth considering. Seems to be "one to watch". Tormead Academics are also sound. Not as good as GHS though.
From Website:
2021 A-levels A*-A = 79%
2021 GCSE 9-7 = 89%
Prep school is known for being incredibly nurturing and caring. Has a good reputation for pastoral care in the area and for teaching the girls about kindness, alongside other life skills, such as confidence, social, learning from mistakes, etc.
Academically the prep does well enough that they have scrapped the entrance exam from prep-school to senior school, as they say their initial assessment on prep entry and academic focus through the prep years is good enough to get girls ready for their senior school (over the road) and achieve the headline results above. Worth noting i've known girls who couldn't get into GHS Junior school, go to Tormead Prep, and then successfully pass the entrance 11+ into GHS for year 7. So although I guess Tormead definitely doesn't want to loose the kids(!!), ironically it does prove that what they do there produces well rounded, academic enough girls... who can go anywhere.
Positives:
Only one entry stress required for your DD all the way through to sixth form, as it's is a through school and like I said, no internal 11+. Decent academic numbers at the end of GCSE and 6th Form seem to be the norm on their website. Good pastoral care rep. Same goes for all the through schools, she will gain a good selection of mates that she will carry with her through the whole school 4-18. Extracurricular offering seems to be expanding rapidly too, compared to what it was, which was pretty much just gymnastics only (although they are amazing at it). Website seems to hint at discount for second DD, I think when she is old enough, if it's right for her too... Also right next door to Lanesborough (RGS Prep) for your DS also, good for drop offs. New head seems to be making positive waves. Fees on the whole lower than the others.
Negatives:
Much smaller site than some of the others you have mentioned, less countryside fancy too, although smart enough and well thought out. Not as much historical investment in facilities as GHS, (however friends say they have just started investing in the school, inc prep, which was part done over the summer). About time as some areas were starting to look tired. Sixth form quite small, I think they go from say 100 in year 11 to 50 ish in sixth form, so loose about half to (usually) co-ed sixth forms....Although, last year my friend's daughter moved into lower sixth and there are over 60 of them. She says staying seems to be becoming more desirable,. Friend also mentioned girls seem to start at another (usually co-ed) sixth form, hate it, and then come back to Tormead after a few weeks. Think a lot of girls' schools see this happening.
Where will you be living (couldn't see if you mentioned that?) It's a big factor with traffic jams around Surrey, the villages and Guildford.