Littleham
Your post refers to her favourite subjects. Clearly your daughter is a gifted all-rounder, and clearly teachers in different departments want to teach her. However you seem to be starting from the premise that she takes her favourite subjects at A level and then continues with one of them at University.
There are lots of threads about state/private University access etc. I don't know if this is an example but most of DC's contempories will have considered life after University. This does not necessarily mean linking University study directly with future career. Some will have decided they are humanities/liberal arts type people and will want to pursue that avenue at University. Others are gifted at languages but talk about following this with a law conversion course. Chemists consider banking/management consulting as a final destination if they don't find career opportunities within their field. Some will have a passion for philosphy or history of art. Though the vast majority seem to choose something quasi-vocational such as engineering, law, medicine, economics or computer science.
These are lucky kids in selective London private schools. Even if their parents are not obviously sucessful (us), their contemporaries have parents who run things or are well known in creative or other professions. Everything seems possible, so you aim high and treat A levels as building blocks. You consider which subjects hold sufficient interest to allow youself to immerse yourself in them at University for 7 days a week for three years. (Hence my earlier comment about maths. There are lots of degrees with a high maths content but you need to be passionate about maths itself to read pure maths.) You balance how well you are likely to do in a subject (and interest obviously plays a big part), the prestige of the University you are likely to get into (so you might be better off reading MFL and going to a "better" University and doing well followed by a law conversion course, thangoing to a less prestigious University to read law) and the type of University (Campus, City).
What really interests your daughter? Will she read popular articles on new developments in, say, neuroscience, what documentaries does she watch, is she fascinated by German society and how it differs from the UK, are tickets for a classical concert her ideal birthday treat?
Failing all else could you try some career profiling? DD had one done through school as part of choosing A level options by a firm called Futurewise. All it did for her was confirm that her academic profile and her interests matched her chosen career, but other parents seemed impressed, with one or two surprising ideas being thrown up.
Once you have decided on a University subject/area and have considered the doors that such a subject might open, A level choice becomes easier. Your daughter is lucky in that it sounds as if she could do most subjects. If say, she decided neuroscience is something that really interests her, she looks at degrees and works back. So perhaps Maths/Chemistry/Biology and one of Music/German. She then finds an outlet for her music, or finds a private German exchange. (If you can afford it I can recommend Geothe Institut summer camps. Three weeks language in the morning and well run activities in the afternoon. She would then sit an EU language qualification which is acceptable to UCAS.)