Most of what I've heard from those with the detailed mod tools is that adding settlements is NOT actually possible. On the other hand, I'm of the opinion that Britain's vast mineral wealth in coal and iron (France, meanwhile, historically was chronically lacking in coal deposits), deserves them a second iron mine, of either high or abundant yield. So, before I write it off, I just want to make sure it's impossible.
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(Is it possible to give a specific mine a yield above abundant, but not others? Don't think so, but I want to be sure.)