The ‘discourse of Britishness’, which proclaimed the ‘natural unity of the island(s) of Britain, inferring from physical continuity and inevitable political unity’, was a powerful ideological force in the twelfth and thirteenth centuries, and was used by both the Welsh and the English to justify political action. For the English, it culturally legitimised the English seizure of Welsh territory, which culminated in the Edwardian conquest of Wales in 1282–83. For the Welsh, it galvanised resistance to English hegemony. The Welsh text Enweu Ynys Prydein, or Names of the Island of Britain, unambiguously proclaims the right to Welsh dominance over Britain. It yokes together the whole island: ‘Teir Ynys Prydein: Lloegyr a Chymry a’r Alban’ [‘Three Realms of Britain: England, Wales, and Scotland’] and continues: ‘Ac nyt oes dlyet y neb ar (yr) Ynys Honn, namyn y genedyl Gymry ehun, Gweddillyon y Brutannyeit, y ddeuth gynt o Gaer Droea’ [‘And no one has a right to this Island except only the nation of the Cymry, the remnant of the Britons, who came here formerly from Troy’]. While Geoffrey of Monmouth's Historia regum Britanniae is significantly more ambiguous than the Enweu, it is possible that in addition to the pseudo-Nennius's Historia Britonnum, he drew on similar Welsh oral traditions, especially the Trioedd Ynys Prydein, in espousing a mythical paradigm for the unity of Britain: ‘Insula hec Britones et Pictos et Scottos incolas recepit. Britones autem a quibus nomen accepit in primis a mari usque ad mare totam insulam insederunt.’ [‘This island received Britons, Picts and Scots as settlers. However, the Britons from whom it first took its name occupied the whole island from sea to sea.’] Geoffrey's text has been interpreted as both an argument for the restoration of Welsh cultural pride and as a contribution towards the ideology that validated English domination over ‘totam insulam’, the whole island.

The Gough Map of Britain, too, can be read as a participant in this discourse: while it is a matter of some debate, it has been interpreted as a mappa regni, a survey of Edward I's empire or latter-day Domesday Book, reflecting his claim to the whole island.