If I might chuck another school into the mix...Alderbrook
I've been the head teacher for the past year and, while having a very obvious bias, think it is a great school!
Ravenstone and HC are also really good schools and I say that on a personal and professional level. As you have said, visiting open days is really important as that is the only way you can really get a feel as to which is the right school for your child.
As for places, you can always ring the respective school offices or Wandsworth Pupil Services and find out if there are any spaces in Year 1. There is always a bit of a move-around at the start of the term; call later in September for a more accurate idea.
And good luck with the move!