In London there is a wide choice of Preps and approaches perhaps reflecting the wider variety of 11+ and 13+ testing. Different parents will have different priorities so it is important to choose carefully.
A co-ed prep has a problem. Girls might stay on till 13 to board, but most will take 11+. 11+ varies so Wycombe Abbey, say, tests on subject knowledge, whilst others might test a combination of maths, english, VR , non VR and, in the case of SPGS, science. Grammar schools have different tests altogether and it is strictly marks in the exam. . The academic private schools are looking for potential, and children who will thrive within the particular ethos of their school, so interview and school report are important.
Boys then either head for 11+ or 13+. And even at 13+ there either school exams or Common Entrance, the latter being quite a difficult test of knowledge across a number of subjects. (At the prep they suggested that the reason why senior schools got such impressive results is because they were taking kids who were already close to GCSE level at 13.)
My kids prep was quite focussed on 13+ and did not separate out the 11+ candidates. This generally did not matter as the local 11+ senior schools could use the combination of results, report and interview and make a judgement on potential. DD tried some less obvious schools, and she did quite a lot of exam practice at home. It might have been easier to send her to one of the Central/West London girls preps who are very good at prepping for 11+ and getting girls into academic schools. I am glad we did not, despite having to provide extra support at home. She started in Yr 7 way ahead of the others in a variety of subjects which provided her with a really useful confidence boost. She was also perhaps more engaged in education than had she spent significant time at school practicing VR and non VR. People looking at Grammar school also had to prepare outside.
So a case of Caveat emptor. If you see the primary function of a Prep school as getting your child through to the next stage, find one which feeds large numbers into your chosen secondary. If you want something more child/education focussed and are content to make a later decision on where the child goes onto, then choose your Prep on that basis but keep an eye out for any gaps in secondary school entrance requirements, and discuss these with the school early on. (The school were helpful in finding the extra material for practice at home and gave her some extra lunchtime help but would not have been able to set aside class time specially for her.)