I was thinking that the official Brythônik wouldn't develop mutations (partly because latin partly because I don't want to bother) but that it will creep into a few dialects, namely areas in the north that are close to 'Cumbria', and those people who interact the most with the Irish.
Also, I've been working out the titles and names I want to use. Tell me what you think
Naming system
Baptismal name>Lûith (clan) name>Family name
Arthur>Auriad>Pendraig
Noble titles
Aurling titles
Nobilissimus Caesar Occidentalis
Imerâdôr o Brythain (not recognized by the Coelings or Brynicians)
Âlt Rî o Brythônys
Bretwalda of Englisc
Subordinate titles
Pendefig o X Lûith ~ Dux Armoricia ~ Cyning of Engle/Seaxa/Iute Cynn