I have never heard of any employer taking the slightest notice of a graduate's A-level grades. Most application forms don't even ask for them. Your class of honours achieved for your degree studies will, almost certainly, be the only academic qualification that any employer will look at.
So I think it's completely pointless to retake your A-levels. However (with the exception of syllabuses that require continuous assessment throughout the course) anyone is free to take any A-level exams if
(a) they can find an examination centre which accepts external candidates (FE colleges normally do; some schools might not) ; and
(b) they're prepared to pay the relevant examination fees.
So, if you can find a college which offers evening classes, or pay for private tuition, or study the relevant subjects on your own, you can retake your A-levels. But I still think that you'd be wasting your time.
University should be fun. You do enough studying already. Get out there and enjoy yourself. If doing so involves, for example, working with disabled children or simply challenging yourself by climbing a few mountains, it will count for much, much more (when it comes to impressing prospective employers) than any A-level grades ever could.
Chris