OP,
I'm the mum of a successful reapplicant. . He was pooled by Oxbridge last year but didn't get an offer.
He got pretty good A levels. Others on this thread have DCs who did better, but he got A* in the subject he wants to read and top levels within that. He also wrote a good personal statement. I don't know what the school said about him but they have consistently underestimated him.
One interview went well. The other was a bit iffy, by all accounts.
The extra year has made him better at his subject. He continues to read in it. His gap year includes one thing which is relevant and was mentioned in his personal statement.
He has also gained maturity. He has had offers from two other top RG HEIs and was fully expecting to go to one of them, with pleasure and grace. I.e. He had recognised that he might v well not be accepted by Oxbridge or indeed one of the other v highly regarded HEIs, specialist in his subject , because competition is so fierce from both UK and international candidates.
If you think your DC can cope with rejection twice, and can organise a worthwhile gap year, no problem reapplying. A year at their age makes little difference. . If not, don't waste time and get on with life.
What's my qualification for saying this? Well , apart from being a mother, I used to be in touch with the senior admin of quite a few HEIs for my job. In general, the feeling was that it was easy to spot the top five or ten per cent. Thereafter there were so many good candidates it was difficult to make a choice. I can't imagine things have changed.