Dunno, Mananan, but welcome aBoard. This isn't all that helpful, but:
Britain
1297, Breteyne, from O.Fr. Bretaigne, from L. Britannia, earlier Brittania, from Brittani "the Britons" (see Briton). The O.E. was Breten-lond. If there was a Celt. name for the island, it has not been recorded.

British
O.E. Bryttisc "of or relating to ancient Britons," from Bryttas "natives of ancient Britain" (see Briton). First record of British Isles is from 1621.

Briton
Anglo-Fr. Bretun, from L. Brittonem (nom. Britto, misspelled Brito in MSS) "a member of the tribe of the Britons," from *Britt-os, the Celtic name of the Celtic inhabitants of Britain and southern Scotland before the 5c. Anglo-Saxon invasion drove them into Wales, Cornwall, and a few other corners. Only in historical use after O.E. period; revived when James I was proclaimed King of Great Britain in 1604, and made official at the union of England and Scotland in 1707.
etymology

I checked an Aussie slang dictionary, thinking the pejorative meaning may have been there due to all the prisoners, but it wasn't listed.