Britain 1. a. The proper name of the whole island containing England, Wales, and Scotland, with their dependencies; more fully called Great Britain; now also used for the British state or empire as a whole. After the OE. period, *Britain was used only as a historical term, until about the time of Henry VIII and Edward VI, when it came again into practical politics in connexion with the efforts made to unite England and Scotland; in 1604 James I was proclaimed `King of Great Britain'; and this name was adopted for the United Kingdom, at the Union in 1707. After that event, South Britain and North Britain are frequent in Acts of Parl. for England and Scotland respectively: the latter is still in occasional (chiefly postal) use. (So West Britain , humorously or polemically for `Ireland'.) Greater Britain is a modern rhetorical phrase for `Great Britain and the colonies', `the British Empire', brought into vogue in 1868.
I know the official use of "Great Britain" (= England + Wales + Scotland) dates back to 1603, when James VI of Scotland also became James I of England (and Wales). The actual union of the kingdoms (of England(Wales) and Scotland) came a century later, with the Act of Union of 1707.
a855 O.E. Chron. Introd., Gaius Iulius se Casere ærest Romana Breten-lond δesohte. c890 K. ÆLFRED Bæda I. i, Breoton is ealond. 1297 R. GLOUC. 22 And aftur Brut ys owne nome he clepede hit Breteyne. 82 Bretayne. a1375 Joseph Arim. (Vernon MS.) 232 {Th}e Auenturus of Brutayne. c1428 Arthur 265 Maximian kyng of Bretaingne Conquered al France and Almayne. c1500 Lyfe Jos. Armathy (W. de W.) lf. 4 Ioseph of Aramathia..came in to grete Brytayne. c1505 DUNBAR `Schir for {Ygh}our Grace' 11 Fairest and best In Bartane. c1515 Prophecy of Bertlington, The French wife shal beare the Sonne Shal weild al Bretane to the sea. 1542 HEN. VIII Declar. Scots Bivb, Brutus of whom the realme than callyd Brytayn toke fyrst that name. 1547 J. HARRISON Exhort. Scottes Hvj, Ye names of both subiectes & realmes ceassing, & to be changed into ye name of Britain & Britons, as it was at first, & yet stil ought to be. 1548 N. BODRUGAN Epitome Avb, England the only supreme seat of thempire of greate Briteigne. 1604 Procl. Jas. I, 24 Oct., King of Great Britain, France, and Ireland. 1630 WADSWORTH Sp. Pilgr. vii. 69 His Majesty of great Britaine. 1665 MANLEY Grotius' Low-C. Warrs 779 King James..obliterating the names of Scots and English, would have both to be united and grow up into one Kingdome..to be called Britain. 1667 DRYDEN Ann. Mirab. Ded., To the Metropolis of Great Britain, the most renowned and late flourishing city of London. 1707 Act of Union xi. §1 That the two Kingdoms of England and Scotland shall..be united into one Kingdom by the Name of Great Britain. 1710 Act 9 Anne vi. §4 To export and transport from Great Britain into Ireland. 1718 Act 5 Geo. I, xi. §16 The importation of Tar and Pitch from North-Britain into any part of South-Britain. 1729 Act 2 Geo. II, xxxv. §12 In several Parts of North Britain commonly called Scotland. Ibid. Brought..to that part of Great Britain called England. 1740 THOMSON `Rule Britannia', When Britain first, at Heaven's command, Arose from out the azure main. c1800 DIBDIN `I sailed from the Downs', So adieu to the white cliffs of Britain. 1832 Act 2 & 3 Will. IV, lxxv. §1 In that part of the United Kingdom called Great Britain, and..that part of the United Kingdom called Ireland. 1868 C. W. DILKE (title) Greater Britain: Travels 1866-67.
Having taken a look at Chaucer (late C14) I may add that he (or his characters in the Canterbury Tales, who use different idioms according to rank, regional origin, and personal foibles) uses the following:
Britaigne, Britayne, Briteyne, to mean either Britain or Brittany (which he also calls Armorik(e): "In Armorik, that called is Britayne" Franklin's Tale, l.1);
Bret, Briton, to mean Welshman;
Britoun, Briton, to mean Breton.
No use of (for example) "Grete Britayne" to mean "Great Britain".
