I have experience at both Rowan and Milbourne.
DD1 went through Rowan, she was happy, however for a private school, it isn't very pushy on the academics. Individually there are some outstanding teachers, but if you are unlucky, you could pass through the school never meeting one of the super star teachers. There have been changes of head, key changes of staff and I don't know anybody that was happy with their final senior school destinations that didn't tutor their child. That said, it has lots to offer across the board and a majority of leavers are happy with their destinations which I think is about what you would expect to hear for a non selective (at pre prep) intake. Typical destinations:
LEH, GHS, St Catherines, Wimbledon High, Putney High, Surbiton High, St Johns (Co-ed), Tormead, St Teresa's, Notre Dame, Manor House, Tiffins Girls and more recently lots of good things being talked about at Claremont (Co-ed). A few girls go off to boarding schools. There is no real 'feeder' pathway to other United Learning schools in my experience. My observation is that the girls that head off to senior schools at the heady heights at the top of the league tables were always destined to and the school has had very little, if any influence on that. Additionally GHS opened up it's intake a few years ago so there are almost twice as many places up for grabs, so more girls going to GHS from Rowan more recently doesn't mean the school is developing more capable girls. It is simply a reflection of a huge increase in available places for 11+ entry at GHS.
DS2 went through the pre prep at Milbourne. The teaching is outstanding. The pastoral is a bit marmite. It does have an old fashioned feel, yet there is always something a bit Harry Potter about the place, which really suits some children. We moved on at the end of Pre Prep. The intake of girls in his year was almost 50:50. There is no significant expansion of the year group at Y3, still two classes, the intake at that point comes from the spaces that open up with the significant minority that leave at the end of Y2 for RGS Guildford, Kings (Wimbledon), St Paul's Boys. It's typical for a few of the girls to leave for LEH & GHS at that point too. One reason the boys might win more competitions is that all of the girls leave at Y6, I can think of possibly one exception in my time at the school. Many boys choose to stay on until Y8. Milbourne is selective at pre prep entry unlike Rowan.
Other girls and Coed preps in the region, Felton Fleet (Coed), Surbiton High, Notre Dame, St Teresa's. Not to be forgotten, Claremont (Co-ed) which is on the up according to everyone I hear speaking about it.
There is a huge amount of choice in the region, I've probably missed off some really lovely schools. Take a look at the Good schools Guide online, visit as many as you can. Somewhere will feel right. There are plenty of options if things don't work out as you would have liked.