Looking at the files it appears you have to be of Briton, Cornish, or one of the other native british cultures (not an Irish or pictish culture. As well as possibly having created the Kingdom of Dumnonia prior to it. Here's the code:
Code:
create_britain = {
only_independent = yes
is_high_prio = yes
potential = {
OR = {
culture = briton
culture = welsh
culture = breton
culture = cornish
culture = cumbric
}
k_britain = {
has_holder = no
}
NOT = { has_global_flag = VIET_britain_restored }
}
allow = {
independent = yes
OR = {
AND = {
realm_size = 80
has_landed_title = k_england
}
completely_controls = k_england
}
OR = {
AND = {
k_dumnonia = {
has_holder = yes
}
OR = {
completely_controls = k_dumnonia
has_landed_title = k_dumnonia
}
}
AND = {
completely_controls = k_brittany
has_landed_title = k_brittany
}
}
piety = 200
prestige = 1000
}
effect = {
set_global_flag = VIET_britain_restored
hidden_tooltip = {
narrative_event = {
id = VIETceltic.0
}
}
}
ai_will_do = {
factor = 1
}
}