In the US, “oatmeal” refers to rolled or steamed oats, the stuff with which you make, well, oatmeal. However, in Britain “oatmeal” refers to what in the US is called “oat flour,” i.e. ground up oats. Rolled or steamed oats are called “porridge oats” in Britain, and our “steel cut oats” are “pinhead oats” there. For a traditional Cranachan dessert, use steel-cut oats.