Wow, that's a terrible wiki section!

"Britain" comes from the Latin "Britannia", which is derived from the Brythonic Celtic term "Pritu" (or some variation of it) which means "shape".

In Welsh and Irish, this term survives as "pryd" and "cruth" respectively. But why do Brythonic and Goidelic terms from which "Britain" is derived start with 'p' and 'c'? When proto-Celtic began to evolve into regional languages, it was split into 2 main groups: P-Celtic languages, which included Brythonic and Gallic languages (with modern survivors like Welsh, Cornish and Breton) and Q-Celtic languages which included Goidelic (the predecessor to Irish, Scottish Gaelic and Manx) and the languages of the Iberian Celts.

Both language groups replaced q/c or b/p sounds with their respective x-Celtic sound; for example, 'son' in Irish becomes 'mac', which was shortened to 'ap' in Welsh. Going back to "pritu", we can tell that it was a name indigenous to Britain because its derivations are found in extant Insular Celtic languages. In early Irish historical texts, the Pictish inhabitants of northern Britain are called the "Cruithin" (just replace the 'c' for 'p' or 'b'), as were a people who inhabited northern Ulster. In the context of Britain, it seems that this term was applied to Brythonic Celts living outside the borders of Roman-Britain. I think that it's more likely that the Irish Cruithin had their name derived from the same etymology (shape or form) than concluding that Picts settled in Ireland (though it's not as unlikely as you'd think).