I would not take a gap year just to apply to Oxbridge. There is no guarantee that you will get in and additionally, the one year lost is really not worth it in the grand scheme of things. The way I see it, if prestige is important (and you will find it becomes less so as you get older) I would do like you said, choose one of these schools and if you want, apply to Oxford in 1st year, if you don't get in, I would just study hard, aim to graduate with a first, and if at the end of your degree you still want to, you can apply to do a masters at Oxbridge. I've looked at a number of resumes and linkedin CVs over the years and honestly, apart from the occasional rooting for your own school, no one really sees universities in anything other than general tiers and any of the universities you mentioned above would give you the opportunity to succeed to your full capacity, you really don't need Oxbridge to do it. If there ever is a difference, it may come in the form of networking and the relationships you build at university, but again the difference even between Oxbridge and the others is in my opinion not worth taking a gap year to take a chance on.
Another thing, don't fret too much over your choice of school. It seems like a big deal right now, and it is, but there are so many other ways to distinguish yourself. Your internship, awards, scholarships, extracurriculars all play as big if not a bigger role and each year you step away from university the less and less your university matters.