Is she only interested in ballet? Have to say that she will be facing something of an uphill struggle at this point - it is possible to get into schools at 16, but most who do have been doing many hours training a week for years. They have to be at the same standard as those who have been in full time training since they were 11, and that is really tough. It isn't impossible, but I'd say she would need to have a pretty much perfect ballet body (proportions, flexibility, feet, etc etc) to have a chance, and would also need to drastically increase the number of hours a week she is dancing.
Has she done any summer schools, or anything like EYB or NYB? Things like that can be a good way of gauging your level against other young dancers. How were her auditions last year? Where did she apply and how did she feel they went? Did she have any recalls? Is she/has she ever been on any associate programmes? Or even your local Centre for Advanced Training? I have to say, virtually all ballet students who get places at 16 have usually been part of some type of associate scheme - RBS, Elmhurst, London Senior Ballet, etc etc. But you can only join these in September, having auditioned the previous year, so that would be no good until September '13, by which time she would be 17, and I'm not sure if they take them on at that age.
In the immediate future, I would look for more classes. Advanced 2 would be good if you can find them, but at this level it is more important to get used to picking combinations up quickly and performing them well than it is to learn and polish a syllabus over months/years. Try to find some advanced non syllabus classes for her to attend, and if possible some Adv 2 as well.
Would Byfleet be too far for you? Susan Robinson is the first school that springs to mind when I think of very high quality non vocational schools. They certainly do Advanced 2. There are others around though. Look at where finalists from things like Young Dancer of the Year are from.
Sorry for sounding so negative. It isn't impossible, and some schools (Central, London Studio Centre, Northern) will accept 18 year olds, so it isn't necessarily too late. But it is incredibly competitive, and unless she has been doing things like associate schemes, summer courses etc, she will be at a disadvantage. Is she interested in any other types of dance? If you're looking at contemporary or musical theatre then completely different rules apply, and it is far less age restricted - she'd still be very young in those terms. Ballet is brutal though - the kids who don't make it in at 11 have to then keep up with those who are training full time, as well as hoping they don't grow too tall/have wrong feet/have the right size head/flexibility/all sorts of other stupid things. Those who haven't been dancing for lots and lots of hours a week and doing associate programmes etc have that to catch up with too, so it really is tough. But it can happen - you get someone like Melissa Hamilton come along, who has had far less training pre-16, gets a place at Elmhurst, gets told she will never make it a year later, and a couple of years after that is a Soloist in the Royal Ballet!