Well,
I haven't found the event that spawns the Romano-British in any text file, so I must assume that it is hardcoded.
The problem is now, that I couldn't remove it yet, which is very bad for modding.
I already tried removing the "spawned_on_event" flag in descr_sm_factions.txt but it didn't work
I then tried putting them on the map in the beginning, which didn't work, either.

Did anyone manage this yet? If so, please tell us how you did it.
I'll continue research on this, too.