my older dd is at BHS senior school. I have heard nothing but praise for the junior school. Its currently no11 on the UK prep school league table. its highly academic.
Ashgrove is also a very good school from what i have heard.http://www.thesundaytimes.co.uk/sto/Parent_Power/
Ashgrove is smaller than BHS in terms of nos but am not sure about results in Ashgrove although my friends ds that goes there really likes it.
From what i hear as well, BHS expect most of the girls to go on to their senior school so you may not get help in preparing for the 11+ if thats the route you want to take for senir school but i am aware that majority of the girls that leave BHS junior school end up at Newstead (is this down to the teaching at BHS or input from parents; i am not totally sure)
Ahgrove will probably prepare your dd for the 11+ as they do not have a senior school.
BHS also has fantastic sporting facilities and a well developed arts, drama and music department. Ashgrove do not have as much facilities as BHS due to the size but that depends on what your dd enjoys doing.
BHS is also girls only and Ashgrove is mixed, do you mind this from an early age.
Bromley mum has a dd at BHS junior school so hopefully she will be able to give you more info.
I think you need to think about size, academic results, facilities depending on what your dd enjoys doing and girls only or mixed at a young age.