Re-visiting this thread now that I'm in front of my computer. (For background, I worked in a Magic Circle, then international, then ££££ US firm, and now work for myself helping graduates land legal and other jobs - so I see 3-4 candidates at this stage daily, which obviously informs my views about what they should be doing to maximise their chances of securing a training contract.)
You don't need to be a superstar to land a "Magic Circle" or equivalent job, though some firms will get much more excited about 1:1 candidates or those with strong language skills than others. What you do need is a sustained, demonstrable commitment to law and (barring exceptional circumstances) a 2:1. Yes, candidates with 2:2s land training contracts at all sorts of firms, but it's more difficult to do so.
At your daughter's stage, she shouldn't imo be thinking about paying for the GDL - she has enough time to apply for roles in a way that will secure her sponsorship from a firm. The GDL is near-useless unless candidates go on to train in legal roles, so going into it without a legal role lined up isn't a good idea in my experience.
What she should be doing now:
Getting in touch with some of the law firm "campus ambassadors" (there are lots at Oxford) to ask about upcoming events - law fairs, meeting firm reps, debates, commercial talks, evenings out. She should get these in her diary, attend them, and use them as an opportunity to learn more about life at these firms and in particular practice areas. (She should also try to go in with her eyes open - if what she learns is that Firm X requires all trainees to spend a year doing mindless corporate fact-checking daily from 9am to gone midnight, is that something she's happy with for the money? What does she want her career to look like in 2,5,10 years?) When/if she applies for these firms she's in a much better position for being able to say that she met Sally from HR and trainee Bob at event X and enjoyed speaking with them about Y.
Getting legal work experience - approaching firms close to home for holiday experience, even if it's just shadowing staff and making the tea. It's good for her CV and to build up her understanding of what lawyers in this or that field do day to day. Also learning key dates by which she needs to apply for spring/summer vacation schemes with the larger law firms for 2019.
Getting other work experience if she doesn't have any - any role can be useful - working at Tesco develops customer service skills, waitressing shows ability to work under pressure and maintain high standards of service etc, some admin jobs build attention to detail.
Researching pro bono or general volunteering roles she can take on - again, useful skill building and great for her CV.
Joining legal and related clubs and societies.
There's a lot more but that would be my initial advice.