We conduct an analysis of the link between colloquialisation and democratisation in debates in the British parliament. Our corpus is a sampler of the Hansard archive, covering the period 1803–2005 and containing 170 million words. We first investigate how the linguistic patterns of parliamentary debates have evolved, and second how the content of the debates has changed over time. Combining these two research questions allows us to step to a third question: are there direct relations between linguistic features, most notably colloquialisation and compression of language, and the topical content of parliamentary discourse, particularly its underpinnings in social and political democratisation processes? We also critically discuss strong correlations between features of language complexity and democracy indices. We adopt an interdisciplinary perspective embedded in both linguistics and political sciences.