Carl, I would go with her instincts.
I don't know if I am an interloper or not, as DD has a deferred place for 2017. She was applying for a really competitive course where there was no guarantee she would get a place, and illness meant she had skipped the medicine equivalent of ELAT so was limited in where she could apply. Yr13 was tough anyway as everyone was stressed especially those applying to high profile Universities in the US and UK. (Even if you were reasonably laid back the stress seemed to be contagious.) She had switched schools for sixth form, loved her new school and wanted to eek out every last minute. She also was not interviewed till March so did not get an offer till early April, so it went on forever, with a huge pile up of course work, revision and interviews just before Easter.
So for much of the year she was working on the basis that her UCAS application was a trial run, and that it was fine if she ended up taking a gap year and reapplying. In the event she got her first choice, but then decided she wanted to take the gap year anyway. She was tired. Simple.
Its working out fine. She has not envied her friends heading off all over the place, and instead has had fun visiting them, or seeing them when they are back in London. The switch from school to university has been tough for some, and I am sure she will start better prepared.
The one rule was we were not prepared to have her at home doing nothing. Instead she has done an eclectic mix of helping out with a residential week for disabled people, worked on a market stall, and is now interning at a cookery school. (In a nod to another thread, this has been a real eye opener in terms of how hard people running small businesses work.) Next will be a ski season as a chalet host and then hopefully Camp America, possibly one for people with disabilities. She has also kept up sport and volunteering.
She has started to say she is looking forward to getting back to studying. That was not the case in September. I think she will arrive at University far better prepared, with better organisational skills, wider horizons, and a better self-understanding of why she is there. Plus her cooking has improved, as has her washing up, and she has a good opportunity to improve her French and skiing.
I have enjoyed having her around during this transition year. The next five years will be a slog, more so for her as she is dyslexic and taking a course that will involve a lot of reading, so I am glad she has had a break and a chance to find herself.
Not getting Oxbridge can be a biggie, even if a DC was not too bothered whether they go there or not, especially at a school where some measure results by the number of Oxbridge places. For DS it was his first taste of "failure", and it hurt more than we expected it to, and at a time when he needed to focus on the last two terms at school. I would leave it a year and use an Oxbridge/other UK application as a Plan B should the Plan As not work out.
Anyway forget the DC, twins and double/triple applications plus the other joys of Yr13 would be too much for me as a parent.