I am surprised at a lot of these replies! While I do think gap years can be really useful for lots of reasons, I can 100% see why you are concerned OP, as the reason she has given 'might possibly get onto Oxbridge next year,' isn't as inspiring as 'being more mature,' or 'saving some money.'
Surprised so many people are handwaving the extra fees - yes it's not like an any other debt, but there is a huge difference between approx £100 (presuming she gets a fairly good job) coming out of your wages for ten years, while you are still young, happy to live in houseshares, etc, than for £100 to come out of your wages for the rest of your life, including when you're trying to pay a mortgage, bring up children, etc.
I also disagree that before uni is the only feasible time to take a gap year - I know loads of people who took one straight after uni, also lots of people of varying ages who took time off to travel either between jobs, or as a career break while their job was kept open - more and more companies are offering this. She will also have nearly 4 months holidays each year, both this year after finishing A levels and during each year of uni - adding up to a whole year between them. Again I know lots of people who did all the 'gap year' activities, including travelling, working Camp America or similar (3 months of work and still 1 month left just to chill!), or just working full time to save money for the rest of the year, in these summer breaks.
She doesn't have to make the decision right now does she? I would get her to accept her place and plan a really fulfilling summer, either travelling or working as many hours as she can - basically leave it as late as possible to defer. If she's in the middle of A levels now it's understandable she might want a break from full time education - in September after 4 months off and if she sees all her friends going off to uni she might feel differently.