Consider using the additional business case guides

[cioportfolio]

HMT have published guides which focus on the stakeholders and the decision making process (https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/749085/Programme_Business_Case_2018.pdf and also a shorter one for projects). Not an easy read(!) but section 3 onward provides step by step help which can be used in an agile framework such as the Service Manual, Scrum or similar. In particular, the parts covering an evolving view of the options might be a good fit for this prototype.