Suggestion:
Don't reduce the number of provinces in Britain. But make the 8 provinces poorer - less income. Reflect the fact that Britain was less developed than Gaul before 14 AD (but better than Germania). That way the AI Pritanoi will be geographically safe when they dominate the island, but relatively poor, not an economic superpower rolling in gold.
Even the Romans had a long, tough time turning Britain into a profitable concern. It wasn't exactly the Mother Lode of gold at the end of the rainbow.
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