This final chapter draws on the experiences of other Westminster Model and Commonwealth democracies to consider how a new constitution for the United Kingdom might be adopted. It examines what the process of constitution-building might look like, what institutions should be responsible for negotiating and drafting the constitution and how the participants should be chosen, the process of final adoption of the agreed draft, and the scope at different stages of the process for both political consensus and public engagement.

The design of constitution-making processes has been a subject of increasing academic study since Jon Elster's work on constitutionbuilding in post-Communist Central and Eastern Europe in the 1990s. Elster argues that participants in a constitution-building process may be motivated by ‘reason’, ‘interest’ and ‘passion’, and pursue their goals through a mixture of ‘arguing’ (seeking to persuade others) and ‘bargaining’ (seeking to cut deals with others). He also identifies different types of ‘constitution-making body’ as the main institution in which this arguing and bargaining takes place. The constitutionmaking body can range from a large elected Constituent Assembly to a small appointed constitutional commission; it might also serve as an interim legislature, or be confined only to constitution-making duties.

The constitution-making body may be subject to what Elster calls ‘upstream’ and ‘downstream’ constraints. Upstream constraints are those that apply at the early stages of constitution-building, before the constitution-making body sets to work: they constrain the arguing and bargaining by setting the agenda, by ruling some things in and out, and by determining who is included in the process. For example, an Act of Parliament appointing the members of a constitutional commission and setting out its terms of reference would be an upstream constraint on the decisions of the commissioners. Downstream constraints are those imposed by the need to get approval for the constitution-making body's decisions, be it from the legislature or in the form of a referendum. Occasionally, as in the 1994– 96 constitution-building process in South Africa, judicial certification of the constitution may be required to ensure compliance with conditions set at the upstream stage.