Romano British - (Hard Coded Emergence)
The Romano British are a special case amongst the hording and emergent factions. Although they are not a horde nation as defined in sm_factions, in the vanilla game they do have the ability to survive without settlements. Their emergence is triggered by some hard coded factors; their internal name 'romano_british', the hidden_resource 'britain', and the region name 'Britannia_Superior'. In the vanilla game if all the regions with the 'britain' hidden resource are captured from the original occupier the 'romano_british' will emerge in the region 'Britannia_Superior'.
To use the romano_british as a normal faction starting on the map follow guide here to add the faction to descr_strat. Remove or rename the 'britain' resource in export_descr_buildings.txt and descr_regions.txt. Failure to remove / re-name the resource will result in CTD if the emergence is triggered when the romano british are already on the map.
To make them emerge if different provinces are taken; change the regions which have the 'britain' resource in descr_regions.txt.
To make them emerge in a 'different' region; swap the internal name of the province you want them to emerge in with 'Britannia_Superior'. They will always emerge in 'Britannia_Superior' but location of that province can be moved and its description altered in the text translation file.