Schools vary and so perhaps worth asking them. Almost all, if not all, will have strategies in place to help ensure that talented linguists can catch up, and that less talented linguists get the support they need.
One school we looked had sets in Yr 7 based on "exposure". They said they had a major reshuffle at the end of Yr 7, and a number new to French then found themselves in the top set.
Another had mixed classes. Those that had done a lot of French gained some useful consolidation, confidence, and a pretty easy year. Teaching was at a brisk pace so those new to French had to work quite hard. By the end of the year the talented linguists had caught up and setting was pretty mixed.
SPGS in contrast only teaches French as a second language. So everyone starts with German or Mandarin or something else.
The equally big London thing is how many bilinguals there are. 10% French bilinguals might be a reasonable expectation. Throw in kids speaking a Romance language, eg Spanish or Italian, who have the advantage of both familiarity with vocabulary and experience of a foreign language, and you will see that prep school is only part of the picture. Also not all prep schools teach French well. Some don't at all and others, especially some of the 11+ focussed girls schools don't seem to allocate much time.
In terms of catch up, I think the only thing worth doing might be to take a short trip to France to try to give your child a sense of French as a language, including accent, and an idea that it is worth learning, even if it is just so you can buy your preferred flavour of ice cream. Vocabulary learning in year 7 will be quite tedious so they might as well know what the aim is.