Popping on to say really don't worry about perceived academic differences between LEH, WHS, PHS and other schools. Bright girls do well at all these schools.
DD (1st year Medicine at Oxford) went to PHS and then KCS for 6th form. She had offers from WHS, JAGS and Tiffin. She went to PHS because she liked it most and it offered her a good scholarship. It was also an easy (one bud) commute.
She transferred to KCS because of the academic rigour there - as did friends from LEH, WHS, KGS etc. she was also drawn to the IB. The mixed sixth form offered great opportunities for music and drama and has been a great stepping stone to university. She was also influenced by the fact she had brothers at KCS.
Oh and on the releasing places point, release all places asap. Nonsense to talk about holding onto them for any longer than needed for 'glory'. Have a bit of empathy please.