It depends on the school. I disagree slightly with Copthall. (Unusual as I usually agree.) We tend to know the more central London girls and I would be surprised if many of the SPGS girls we know were not tutored. Some, as she suggests, very very extensively. (The exceptions were very bright and from prep schools.) There is a huge cache around getting your child into the school, and the school is looking for girls who expect to work very hard. The school unlike others, does not seem to do much catch up in Yr 7. You need to hit the ground running. Hence, I assume, their relatively small percentage of kids from state primaries.
The same does not seem to be true of other schools in the area, where the proportions from state schools tends to be higher, and where kids from private preps get the chance to consolidate in Yr 7, whilst the others cover the ground for the first time.
Many kids from state primaries though do seem to have had some tuition. I went to a Yr7 coffee morning where there was a widespread moan about the local tutors and how much they charged. I suspect this may have been the first time people were open about the subject. Many of the kids had had a shot at grammar and were primarily prepared for this, with the indie as a fall back.
Like any exam your child needs to have covered the syllabus and be secure in their understanding. Prep schools should have done this. If your child is from a state primary, there may be gaps in maths which need to be covered and unless your child is a natural, they may well need essay and comprehension practice. They should also be familiar with the sorts of VR and non VR questions that may come up.
Beyond that they need some exam practice, the aim being to have them go into an exam confident and able to perform well in a timed setting. Again prep schools have this covered. If you child has not done many exams, get them to do timed papers. If you have room you might also consider applying to a couple of selective state schools amongst your options. Their exams are in the autumn and will be way more scary. Private school tests, where they lay on good food and try to help kids feel at ease, then appear like a walk in the park.
Beyond ensuring your child is able to show what they are capable of I suspect tutoring starts to be counter productive. Schools are looking for potential. They are judged on what they achieve with 18 year olds, not what their 11 year olds know. Too much tutoring is likely to lead to anxiety or lack of spontaneity, and leave the school unsure how much is the child and how much is the coaching. You want your child in the right setting, and squeezing a kid into a more academic school is probably not in anyone's interest.