Hi Debo, I don't have children at prep school yet but I've heard the following from other parents:
The Granville - good results, very academically focused, quite a yummy mummy "set", some people have said the Mum's are a bit snooty (not my words, I only know one Mum whose daughter goes there and she's lovely).
New Beacon - very results orientated, they expect their boys to be good at something (can be sports or academic, told to me by a Dad at the school). Really push the boys there to do their best. More focused on steering boys to public schools, rather than the 11+ and grammar. Great record of getting boys into Tonbridge and Sevenoaks schools.
Solefields - small school, very family feel, parents really rate it as it's small, friendly and very focused on getting the best out of the child. The parents are supposed to be friendly. People say it's not the academic "hot house" of New Beacon though.
Don't know much about Wally Hall I'm afraid.
Hope that helps some, I have been doing my research too. We were considering a single sex school but have decided to go for mixed, either Sevenoaks prep or St Michael's.