Slitherine

They also are in British campaign,did they had one nation behind both
kingdoms ?It is not unknown in history that some tribe was splitting
at certain moments and migrating.

Dumnonia in Britain became known later-in time of king Arthur,but it is another
queston

It's probably difficult question ,but if anyone knows something about it
would be great to know.

If you'd asked me 7 years ago when I did the research/design I could have told you - sadly it's all gone from my memory now!

Right
Somehow Gaul or South Britain of this era is more known.
(probably, due to the scientists many disagreements and number of Roman sources)

Quote on Cornovii:
*The people who inhabited the very north of the British mainland (modern Caithness), and the English West Midlands ( NW Staffordshire & NE Shropshire) were also known by the same name, Cornovii. In 1973 Oxford University historian Dr John Morris put forward a theory in his work 'The Age of Arthur', that the Cornovii from the West Midlands migrated to Cornwall in 460. There is however no primary evidence to suggest that these tribes were related or that there was any contact between the two and it appears that the only connection is a name similarity. (The names of 'tribes' Dumnonii, Damnonii, Cornovii, Cornavii occur at several locations all over Britain and may simply reflect some language similarity in the eyes of the Romans).
For instance there was a Damnonii tribe in Scotland (the Clyde Valley), whose name looks very similar to the Dumnonii tribe of South West Britain, so many have assumed that they must be the same people but again there is no evidence of any contact between the two*.