Mixed electoral systems seek to combine elements from different voting methods so as to maximise the numbers of individually accountable constituency MPs, while achieving more proportional outcomes in terms of parties' representation. This paper looks in detail at two kinds of mixed systems: variants of the Additional Member System; and AV Plus (or SV Plus). We examine how they would have operated in British conditions during the 1990s—how ballot papers would be structured, how voters respond to them and what electoral outcomes would have resulted. Both approaches offer good prospects for achieving a consensus amongst electoral reformers on an alternative to first-past-the-post elections. We also show how the Jenkins Commission's proposals can be located within the broader development of ‘British AMS’ by the Labour and Liberal Democrat parties. In addition, we quantify the main impacts of the policy choices which the Commission made in designing the proposed system to be submitted to a referendum. The British case will be a key one for the wider debate in political science about the endogenisation of voting system changes within party systems.