She does not need to prove her writing ability in extra curricular stuff to get onto a University English Degree course. As has been pointed out, a classic English degree is more about reading literature (be that ancient, contemporary, British, American etc) and gaining a greater understanding of it - what makes it successful - what are the writers trying to get at and are they successful. What can we learn about the culture and the politics of a society from the literature they produce(d).
Creative writing is something much different, and she may be lucky enough to get on a course that does this as a module. Otherwise, it will be something she works on in her own time, or does an MA in, or a course after she graduates (such as at the London College of Printing).
Getting on an English degree course will be mainly down the the A level results she gets. You could start looking at which Uni's have a strong reputation in this field. Apart from the obvious (oxbridge), places Like Leeds, Durham etc.
The way she will learn to hone her craft as a screenwriter is by reading, reading, reading lots of screenplays. These are available to download off the internet. Sometimes for free, sometimes for a small charge. She should read the good and the bad, and particulary screenplays of films or TV shows she deems to be successful.
Writing for film and tv is an industry many aspire to, but few take the craft seriously enough to really succeed. It's fantastic your daughter has identified a career goal so early, and I would encourage it if you think she has talent because the industry is constantly in need of new voices and fresh perspectives on life, the universe and, you know, everything. 