https://en.wikipedia.org/wiki/Porridge
Oatmeal is a more specific term than porridge, because porridge isn’t necessarily made from oats (eg, rice in Asia).
Though in the UK, when someone says “porridge” they almost always mean oat porridge or oatmeal.
Already the Mycenaean Greeks, 3500 years ago, used a cognate word with the same meaning: "meleuro-".
"Flour", which comes from "flower", originally meant the finest grade of meal, which was considered the best.
Nowadays the usage of these words is not always consistent with their original meanings.
It's not often I find myself quoting Gwar, but this reminds me of "Bone meal! Better than no meal at all!"
A porridge made of rice in the UK is not a thing as far as I am aware (I'm not in hipster London though), I suspect it would be what we call Rice Pudding?
No, for that we've adopted the name "rice pudding"