I just wanted to say hang on in there to those who received disappointing news. I really was sorry to read about your son yesterday, Rejects. I can totally relate as son was also rejected some years ago. At the time, he was very deflated and said that was it and he wouldn't have entertained the idea of reapplying. But as the months went on, he changed his mind. Particularly as, by the time he got to August and A-level results, January didn't seem far off! He was successful second time round and, in my view, there is definitely something to be said for applying with grades in hand. Also, they know the routine and are not in the midst of coursework / mocks while it's all going on.
But... (in my very humble view) I think it's only worth reapplying if they do achieve the max possible grades. I think something like 50% there achieve at least three A stars (the percentage is higher in STEM), so they need to at least have that to immediately present themselves as in the top half of the actual cohort.
Also, if they can do something in the gap year to boost their application, this can really help. Easier with some subjects than others though. Definitely have a plan for Sept - maybe voluntary work abroad or something? But it's vital to have something to avoid them sitting at home seeing everyone else's fresher's weeks social media! As long as they have a structured, exciting and relevant plan for a gap year, it can be absolutely invaluable. Even if DS hadn't got in second time round, he would not have regretted his gap year. He worked overseas in challenging circumstances and, I know it's a cliche, but he was a different person with different perspectives when he came back.
Another thing is to use the extra time to build up some extra super- curriculars such as individual research, entering essay competitions, projects behind the curriculum, etc - because they will expect more from a post A-level applicant (not just the same PS). If they are the kind of student who has that type of stuff anyway, it can be worth reapplying. Perhaps they win the subject prize at school in Year 13, or some other academic achievement happens in Year 13 that just gives their application an extra boost.
Many Oxbridge reapplicants also broaden the scope of the uni search second time round - eg. perhaps applying for some US options? Or universities with dual programmes here and abroad? Sometimes they just need the extra year to decompress and realise that Oxbridge is not the be-all-and-end-all and actually realise (away from all the school peer pressure) that other unis may well be better! It's less stressful for them when they don't even have to let anyone else know where they are applying.
I realise (most probably) will not be interested in reapplying so all this is totally irrelevant (sorry)! At this point, people will be understandably exhausted and deflated and thinking "What more could I have done?" But their feelings can change between now and results day, so I just wanted to mention this because many can (and do) make successful post- A-level applications. If DS' four friends who reapplied, all were successful. But more importantly, all had gap years that were worth it regardless.