Edit 2: Oh, and just to say further, I have seen the AI in the Total War games be incredibly inconsistant. I just played Fall of the Samurai with DarthMod, and the main imperial enemy wasnt any major clan, it was Hiroshima, which became the Hiroshima Imperial Vanguard. In Empire Vanilla, I always noticed that Savoy often defeated France and became a major nation. In Stainless Steel for Medieval 2, I used to fuss about wanting Ireland, and replace the Cumans, since the Cumans NEVER did anything in my campaigns, but for other people, they'd...do something!
Soooo, I'd imagine the AI might would just ignore a one or two region "British Empire" or instantly consume it