If they are not already at a Prep it is much harder to find an selective 11+ place for a boy in West London. Lots of girls schools, but relatively few boys ones. And even some of those have gone co-ed or mainly take at 13+.
Most private girls schools, with the possible exception of SPGS, take significant numbers from state Primaries who will not have had the same level of preparation as those at Preps. At the end of the day they are looking for potential, rather than achievement to date.
Some Preps may claim to have a "hot line" to certain senior schools, but weighed against that is a tendency to make their minds up about girls a couple of years earlier, eg around Yr 4. They can then be quite prescriptive. Top half of top set apply for SPGS/Wycombe. the rest for G&L, Putney and maybe Francis Holland. Second set kids then head for the Queens, More House, St James' etc.
A few years into secondary it is clear that where they were at the start of Yr 7, or where they thought they were, is quite different from where they end up in Yr 11. The girl who shocked that she did not get a place at SPGS has faded, and a couple of late developers have streaked ahead. From observation some Preps do seem better at getting average kids into good schools, but I am not sure it is an advantage to then be at the bottom, rather than towards the top at somewhere less academic.
That said being a Prep means you have a better understanding of where your child sits in the cohort and what to try for. This will be difficult for OP.
Really I would not worry unduly. Make sure schools understand your child's educational background. Use the Galore Parks books to make sure that your child has covered the ground in maths and English. Practice a little timed VR and non VR if these skills are tested. Apply for a few, probably five, and allow for a bit of a range. And then trust the schools to want to take the girls most like to thrive.
In practice schools select, not you. But don't get too put off by the numbers. Most will be applying to five, and big schools like Surbiton will take a pretty broad range.