There is nothing that can be done in the UK . South Africa is the only place it can be reported. If it is was an offence, as I do not know how old the 'minor' was or what the age of consent was in SA at that time. In all honesty I think it will be a waste of time after 15 years and with the change of regime in SA since it happened, the law then was very diffrent from the law now. The only charge that could be brought in the uk ( if it had happened here) would be stuatory rape , but I do not even know if that was a crime in SA 15 years ago.
Whatever is decided, as I said it can only be reported and charged in South Africa not here.