Sophie will claim that she has performed her side of the bargain, and so can sue for the agreed sum. There may be a point of interpretation about whether she had to provide the goods at a wedding, but it is clearly at least arguable that she did not need the co-operation of others in order to perform her obligations under the contract. The real issue here is whether Richard and Julia can argue that Sophie did not have a ‘legitimate interest’ in doing so. This requires you to engage with White & Carter (Councils) Ltd v McGregor  AC 413 and subsequent case law. The issue is perhaps particularly striking here since Sophie could easily have mitigated her loss by accepting the offer from Britney and Jason. You must advise on the positive law, and could also offer views as to whether the law is satisfactory and how it might develop.