Brythonic languages are believed to have been spoken throughout the island of Britain.
From the en.wikipedia.org
Brythonic Celtic is the ancestor of Welsh, Cornish and Breton.
From the thisisbristol.co.uk
Brythonic languages in use today are Welsh, Cornish and Breton.
From the en.wikipedia.org
Brythonic names north of this line are arguably Pictish.
From the en.wikipedia.org
Brythonic as a term is usually contrasted with Goidelic.
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More examples
A southern group of Celtic languages
(also spelled Brittanic): One of the two branches of the Celtic family of languages descended from Proto-Indo-European. Brythonic includes Celtic languages such as Cornish, Breton, and Welsh. ...
Languages were then spoken at least in the whole of Great Britain south of the rivers Forth and Clyde, presumably also including the Isle of Man.