I live in France, teach English in a lycée, and have put two DC through the system, one who is in terminale and just finishing the BAC this year.
If I were you I'd get onto the lycée now, most of the lessons have either finished or are winding down, the terminal students have left and the premiere students are preparing for their French oral exam.
I would strongly advise that you place your daughter into seconde, firstly it will give her a really good feel for the French education system but will also put her in a good position to choose her options in premiere. A BAC generale is not for all and actually there are loads of different types of BAC to choose from depending on her interests.
All schools love to give flannel about how accommodating they are to non-French speaking students. The school I teach in thinks it's amazing, the fact is they may offer some soutien in French but after 6 months they expect the children to be up and running. I hear it all the time, they're using their non-french as an excuse, they don't try and speak French, they should be bilingual by now. I do try and point out to colleagues that they have been working with these children for 6 months and they haven't managed to become proficient in English so why the unreal expectations placed on children?
My advice based on my own personal experience is, let your daughter finish her education in the UK, give her French lessons as an added extra. School in France is not like school in England, it's not a softly, softly approach here. Your daughter could end up feeling really isolated and in fact her education could be seriously affected. My DCs have been in the system since they were 2 and 3 and they always have been told they will never be as good at a subject because they are French, it was brought up in every single parent's meeting, there is no support for non-French speakers and very little for the children with educational needs. My oldest son also went on to have an amazing BAC result with a mention tres bien, it was brilliant. His teachers' comments; well it was because his English elevated his scores (he got 20/20 in every subject) there is a lot of negativity towards non-French speakers in the school system. I don't think it's scaremongering telling you this but I've seen first hand the children who are placed in school here at an older age and some of them really suffer.