Hi there, Im a artist manager from America doing a managers contract with a S African artist whos atty wants it under UK Law will my contact hold?

Yes, you can select the law to be applied to your contract, even if you don't live there. Note that this is a different question from where someone can bring a lawsuit. Any court can apply another court's law, for example you might sue in the United States and the US court would have to apply UK law. Be sure to confirm with a lawyer in the country whose law is to be applied that all of the terms of the agreement are enforceable in that country. You don't want to end up in a situation where your choice of law eviscerates your agreement.