As a wife and mother of Etonians and indeed an old girl of one of the schools mentioned ,heres my two pence worth.
There is no equivalent for many many reasons but the closest (boarding)would be CLC and WA
St Marys Ascot is catholic so thats a non starter.
Benenden-never really features in discussions and when the one thing most people know is Anne went there over fifty years ago..that tells you all you need to know
Roedean,fine,if you want to study Mandarin
Downe House,Heathfield-seen as fallback schools if you cant get into WA
The boys say the WA socials are best ..
Having said the above most of the Eton parents with daughters that we know have the girls at co-eds such as Marlborough and Wellington,two schools that I personally wouldnt want my children anywhere near but each to their own..