I have some experience of this - you wouldn't be asked for higher grades just because you have already taken your A levels. If the offer was AAA and you had obtained AAA, providing your application was good that would typically get you an unconditional offer. The only time this may change is if you are re-sitting. In some cases, very popular, highly selective courses may ask that a student taking re-sits obtains grades one higher than the typical offer, so AAB may go up to AAA - but this is not always the case.
It is unlikely however, for selective courses that they would accept lower grades if you had already taken the exams. The expectation is that you are competing with students who are going to get the required grades so there is no reason for them to accept someone who already has lower.
Does this help? Make sense? I am happy to clarify if not.