This thesis investigates how MPs go about representing the United Kingdom’s component territorial units in the House of Commons. More specifically, it examines how national and regional interests are fed into parliamentary proceedings, how this varies across different territorial, political and institutional contexts, and how the role of the sub-state territorial MP has evolved since the introduction of devolution in the late 1990s.

Before this project there had been no substantial study of how the UK’s component territorial units are represented at Westminster for over 40 years. This is despite transformative changes to the constitutional and political environments in which MPs with seats in Northern Ireland, Scotland and Wales operate. By transferring key policy responsibilities away from the centre, devolution raised questions about the roles of MPs from the affected areas. Political divergence between the different parts of the UK has subsequently become more pronounced, and over the last decade the future of the domestic Union has become an increasingly salient issue.

A mixed-methods approach is taken, combining analysis of 6,001 parliamentary contributions with 23 interviews. It is found that MPs with constituencies in the devolved areas focus heavily on matters specific to their territorial units, and increasingly so since 1992–97. These MPs have adapted their territorially-focused roles to the changed institutional environment, for instance through positioning themselves as champions and critics of the devolved executives and legislatures. This sort of behaviour was especially widespread during the period of intense parliamentary debate about Brexit from 2016–19, although evidence of sub-state territorial representation having influenced the course of these events is limited primarily to second order issues. English MPs are also found to engage in territorial representation of areas larger than constituencies, specifically in relation to counties. However, this is a far more prominent feature of the behaviour of MPs with seats in Cornwall compared to those with seats in Yorkshire.

The trends that have been identified speak to a political sphere in the UK that is increasingly fragmented along territorial lines. These findings carry significant implications for academic literatures on parliament and territorial politics in the UK, and for our broader understanding of the UK political system. The approach and findings also have the potential to inform future research on representation of territorial units by members of legislatures in other multi-level political systems.