[jovrtn]

That etymology for 'Britain' doesn't seem to be referenced anywhere else—the link in the post you've linked to hasn't worked since 2003 and doesn't cite its source either. Seems the name derives from Brythonic (British Celtic) and meant "the painted ones" or "the tattooed folk" rather than a reference to tin. The etymological path it took was (roughly): Brythonic -> Ancient Greek (by way of the Greek explorer Pytheas) -> Latin -> Old English/Middle English/Old French -> Modern English [0][1]

[0] https://en.wikipedia.org/wiki/Britain_(place_name)

[1] https://en.wiktionary.org/wiki/Britain