The way the higher-end universities view re-sits is not necessarily as strict as it's made out to be here. A lot of my friends had to do re-sits and got into good unis, like Leeds, Bristol, Exeter, even Oxford (....Cambridge is possibly the pickiest from what I've heard, but oh well).
That being said...
Say you'd gotten very good grades (A*s) on two of your full A Levels, but the third- which you have no desire to carry on to uni whatsoever- was not so great. Would 'high-end' (NOT Oxbridge) unis frown upon retakes for that subject in January of a post-A Level gap year??