Well then cut britain out? Leaves more provinces for the rest.
In terms of keeping the Casse, it ensures that Britain is not a joke to take as the Sweboz or Gallic factions. Historically, you are correct, no tribe united the isle before the Romans arrived. However its addition imo is merited both to make the isle harder to take and also because British tribes did lend money and possibly troops to help their Gallic and Belgic allies during Caesars wars in Gaul.
And is not entirely true. The importance of a faction after 272 does influence the consideration wether a faction is included or not.
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