How do we sub for British self-rising flour?

That's a great question - something I've wondered about, myself. According to the website of British cooking authority, Nigella Lawson, she "usually suggests using 2 teaspoons of baking powder to each 150g/1 cup plus 2 tablespoons of plain (all-purpose) flour to make self-raising flour." https://www.nigella.com/ask/home-made-self-raising-flour

Hope this helps! ;o)