I would add that by the end of Yr 7, when many schools will set for French, there will be plenty who have just done a year. Schools will expect some to be ahead and indeed DD spent most of Yr 7 coasting. What were 30 basic French words to be read through each week, was a significant homework for others.
I would keep the French to fun exposure, ie so she has some familiarity with basic vocab and the accent. And not worry about Latin. Not that many take it to GCSE, in part because at least a couple of subjects will have to be dropped. So either she loves it and thrives, or she does not and drops it.
As for the rest, yes some prep schools have a head start but honestly all that will happen is that she will have to work quite hard in Yr 7 so might as well have the summer off.
The other big gap will be sport. If she is keen on making a team you might consider joining a club or doing a sports camp somewhere. However LEH is lacrosse not hockey so perhaps not so important as the prep girls won't have played it either. Hopefully sports staff will use Yr 7 to encourage participation, especially amongst those who are not used to school sport. This did not happen at DDs school. The Yr7 netball team seemed to be picked from the tallest of the prep girls. A couple of years later they were having a problem putting out a team because they had limited the potential pool. By then DD had shot up, but refused to be considered. My understanding is that in some schools access to drama and music can be equally competitive, in that you have to audition for plays and orchestras, leaving some "all-rounder" girls without anything.
Also some preps really dont do much except prepare girls for 11+, so very limited science and languages. DD was at a co-ed which went up to 13+, and where they did not seperate girls from boys. As a result she had covered vast chunks of the 13+ Common Entrance syllabus in all her subjects. It was a huge confidence boost for her to suddenly be top of the class, and luckily she decided she wanted to stay there. It could have gone the other way and after coasting for a year she might not have realised when she needed to shift up a gear.