Ah bless you @CandleBlowerOuter 
I'm sorry @clevud I don't know any details about the schools you mention - I gather they are very competitive for sixth form? That's not an issue at all round my way where every student who gets the GCSE grades or even is close will be welcomed into sixth form. What form does the competitiveness take – high GCSE requirement?
I would say it’s fantastic that she has access to two (in fact three) MFL at GCSE and wishes to continue into A level. MFL A level is very challenging, make no mistake. I infer your DD is a native speaker (of Italian?) which is of course a bonus; but even for a native speaker of French, French A level is a challenge requiring hard work, and a top grade is not a given.
Students need to write essays in target language about a film and a book, using excellent language and also good analysis. They also learn about a range of topical and political issues such as immigration and strikes, and have to be able to speak and write about these – so even a native speaker cannot just rock up and do it.
I'm not sure tho that a school would offer places based on whether the A level subjects were challenging; but yes, they would be keen to offer a place if they wanted to run that language A level, as they are notoriously under subscribed, sadly.
Do check that the schools you are interested in will offer the MFL you want. DD's sixth-form cohort at her (non-prestigious I must admit) state secondary was offered only French; DS2's year at the same school offered only German (and it was the only state sixth form setting in the city, out of at least six, to offer German A level that year). And I don't think it is alone in this (tho I suspect it is a different story in the highly sought-after London schools you mention)