I used the regular rolled oats, but if you have to use the instant ones, I probably wouldn't cook them. I'd just mix them with the cream, and let it sit until the mixture thickened up. By the way, this is just a theory, as I haven't attempted this switch.
If you don't have self-rising flour, just add 1/4 cup of all-purpose flour, plus 1/2 teaspoon of baking powder, and an extra pinch of salt.
Feel free to cook the oat mixture immediately or refrigerate it overnight before cooking. The longer you wait, the creamier the insides of the oatcakes will be.