As somebody who has worked in careers guidance for 30 (OMG!!!!) years, and who sent DD to independent school, I would say it really does depend on the individual school.
It would seem that your DD's most difficult decision will be what to study for A levels if she is an academic all-rounder, and hasn't much idea of the direction that she wants to go in. Whichever subjects she chooses will narrow her options & university options. Taking advice from staff who don't know her, her abilities or her interests about this at a new school isn't ideal.
In my experience, using a good psychometric assessment, like the Morrisby Profile, would help her to look at her relative abilities across a number of areas and can match these up with career interests. It is a 3+hour long assessment and combined with guidance from a qualified careers professional can help to choose the best A levels to keep a number of routes open. Look at the Morrisby website to find a qualified professional near you: www.Morrisby.com. They have just released an online version of their test which is shorter, but less in-depth.
The best state & independent schools will be good at preparing students for the top universities, and for academic degree courses. Less good at more unusual or perhaps vocational degrees...They may offer a wider choice of A level subjects, or the International Baccalaureate.
In terms of work experience, most schools only offer 1 week, usually towards the end of Year 12. By then, it's usually to confirm an interest in a career, or to fulfil a requirement for uni courses. Too late for exploration really...
However it's not essential to know what career she wants to go into at this stage. If she's heading for uni; can choose her A level subjects wisely, which will lead on to relevant degree courses.... and then onto relevant careers. Many graduate jobs do not specify degree subject.
If you would like to know more about what jobs different degrees can lead to, look at the "What Do Graduates Do?", and "Options with my degree subject" sections on the Prospects graduate website: www.Prospects.ac.uk.