I teach at a 6th form college and the things that would be important to me are...
What is the average grades for students for each subject that your DD is considering? (Not the average point score per student which a school/college may boost up their scores by making students do General Studies / critical thinking - subjects not included in the offers at Oxbridge, so you need to look behind the headline statistics iyswim!) but look at the results for the subjects your DD will be doing - they can vary greatly, even within an institution.
Also, I'd ask about the syllabus for each of the courses that you are considering - for example, in history which period will they be studying? I did 16th Century Spanish history for my 'A' level and it was sooo hard and boring - is the period right for her? Also, it might be worth comparing the exam boards and looking at what is required of them. For example, in My subject, (RS) we teach EDEXCEL which requires detailed knowledge of a few philosophical arguments, whereas OCR requires students to know lots but not in any great detail. I know some students struggle with the Depth of EDEXCEL, but in the past I have taught OCR and other students have struggled with the 'whistlestop tour' approach of that exam board - different things suit different students, so you need to ensure that what she will be studying will suit her best.
An important thing for 'A' level is the amount of extra support available to students - are there revision classes, one to one support if needed (strangely, I find a lot of 'A' graders at GCSE struggle with the transition from GCSE to 'A' level). Are the students encouraged to speak to the teachers outside of class, or discouraged?) We actively encourage our students to see us outside of class if there's a problem, but other colleges really discourage that.
I'd also look at retention rates and policies - even the best students at GCSE drop out sometimes when the stress gets too much for them. How do they support students, what is on offer for them? Also, what happens if she doesn't like her chosen subjects once she's started them? This happens a lot - is there support if she wants to change etc?
Also, look at Oxbridge / UCAS support of you are thinking of that.
However, the most important thing for me, would be to find somewhere where your daughter is going to be happy studying for the next two years. It is always worthwhile remembering that school for the next two years will be optional, and if she is going to a new school, then her success will probably depend upon fitting in and wanting to be there... I always say to students to go somewhere where they want to be, doing something that that they want to do - because then they will succeed, Whereas if they are unhappy and don't like where they are, that's when students drop out, and leave etc. So for me, this would be the most important question.
One final thing, the school with the highest results may not necessarily be the best. I once taught in a very high achieving school, great results, but the students were completely spoonfed, and were not being taught to be independent learners - no preparation for uni at all. I'm now at a good 6th form college - Good results, but not as high achieving, but the students are more well rounded and better prepared for uni because they are given more responsibility for managing their own learning, and I think they generally thrive on it. (Although it doesn't suit all students of course). But, the 'A' grade student will still get 'A' grades here too - but they also learn so much more, and are more prepared for uni at the end of it. I think there is more to 6th form than just the academic side of things.
However, these are just my thoughts, I've got another 10 years before my DDs choose their 6th forms. And sorry it is so long! Good luck.