And following on from Penguin's point, give active consideration to a gap year as a plan B. University is now a huge investment decision, and you should get on the best course you can.
And a Gap year is often a good thing in its own right, proving a chance to do something interesting and allowing a student to gain extra maturity.
It depends on how competitive the course is (see my post above - it is worth looking at the ratios as this gives an understanding of the University's position.) Then think tactics. DS applied to the four leading Universities for his subject, with the understanding that he would be good enough for any, but that weight of numbers meant that he might be rejected by any or all. In the end he got the University he least expected (they have a reputation for preferring non private school pupils, plus there was a chance that they would not consider two of the five A levels he was offering). If you are a realistic candidate, better to apply for four very competitive courses, thereby increasing your chances of getting on one of them, and take a Gap year than settle for a less interesting Insurance.
Also be aware that at the top end, all offers are likely to be the same. For economics A*AA is pretty standard. So if you are likely to substantially exceed the standard offer, there is little point putting an Exeter or Nottingham down as insurance.
Tactics will be different if you are less likely to exceed the standard offer, but again there is value knowing your grades before you apply.
Also give consideration to London Universities. Many wont because of the expense and complexity of living in London. This means that top Universities (Imperial, LSE, UCL and Kings) are worried about their relatively low numbers of good state educated students from outside the home counties, especially for subjects requiring strong maths/science. If you can make the figures stack up, your chances of getting on a better course may be higher, than say trying for Durham or Warwick.
(On his offer day my son walked across to one of the Halls of Residence with three other lads. Even he was shocked that all of this random group had attended big name private schools of the sort often discussed on MN. Not what I think the University wants, but economic self-selection is alive and kicking. That said, given the majority wont be from the UK, so in practice there will be diversity enough.)