The first, generic, response has to be that until results day you really don't know how things have turned out, so all of this could be needless worrying. It's much better to try not to think about things until you know how you've done, to save on being overly stressed. But I appreciate that's difficult. If your firm university accepts you, definitely don't do any retakes - A-levels are very much a means to an ends in terms of getting into a good university, and whilst employers will look at them it is only your degree grade which really carries any weight (although it may be worth asking why you've underachieved this time).
If you do miss your offer, there's no shame in retaking over the next year, provided you can offer a lot of extra work experience from that time and show the year has been worthwhile. Should you miss your offers, then you should be retaking. But as I said, try not to worry until you know how you've done.