A common AH Cliche, particularly in timelines or maps created by Americans. A state called “Cornwall” is shown extending well into Devon and Somerset and often all the way to Bristol. This despite the fact that in OTL Cornwall is defined by the small area that now exists, and if it was that large then it would still be called Dumnonia and would most probably not be independent from a wider Brythonic state anyway.
A variation on this is to assume that modern Cornwall has a serious Celtic identity or separatist movement, when in fact it has three blokes with St. Piran bumper stickers campaigning for slightly more regional autonomy. The Cornish language died out some years ago and has since been revived, but as no-one can agree on which dialect is the 'right' one, there are actually three sets of Cornish languages. (Perhaps a relation to the fact that it's three blokes ?)