I have friends with DC in The Brit school, both in KS4 and KS5.
I take it you know the school has massively cut back on the GCSEs and A-levels it offers? At GCSE now they literally just do maths, English and science. At A level I think they offer 4 options. I'm not sure why they've done this, I guess that's for you to ask them.
Having said that though, if you look at their results, they get higher than the national average percentage for 5's in maths and English so they're clearly doing something right.
My friends with DC are there because they live and breathe their 'strand'. If they weren't there, they probably wouldn't be in a school because all they want to do is their specialist subject. I believe the school does have quite a high proportion of students who struggle with more traditional subjects (either through barriers to learning or because they're just not interested!).
One of my friend's DS was desperate to go for Yr10 but my friend felt the curriculum was too narrow and it was too much of a gamble. The compromise is that he's applying for post-16.
I think you have to be pretty driven to go to the Brit school as well as being hugely talented, obviously. If your DD is undecided about possibly wanting to get into something like set design, as Newgirls says, it may be better for her to keep her options open at this stage?