It is true that no woman has perfectly symmetrical breasts. However, there is a point where the difference in size and shape becomes, first noticeable to the patient, and then to others, causing distress. The asymmetry may also involve the chest wall (pectus excavatum and pectus carinatum).
The exact procedure will need to be customized to the patient and may involve augmentation, reduction and very likely an uplift or mastopexy on one or both sides.
The patient is quite likely to have a preference for one breast over the other and want to match the other one to it.  A common abnormality is the tubular breast or conical breast where there are varying degrees of large nipple mound and constricted base to the breast.
Surgery offers a good solution to the problem of uneven breasts and understanding surgeons at the Cadogan Clinic will need to discuss your precise requirements.