When a recipe (British, typically) calls for caster sugar, under what circumstances is ordinary U.S. granulated sugar acceptable?

I've found grinding evaporated cane juice, the unprocessed granulated sugar, is the closest thing you can get to castor sugar in a recipe.

I think for some cakes and cookies with a particularly delicate texture, you might get slightly better results if you use superfine/caster sugar. It's really easy to grind granulated sugar into something like superfine with a food processor.

You can also substitute castor sugar with superfine. But superfine and granulated are also a 1:1 substitution. Superfine is good to use in beverages when you want the sugar to be really dissolved. But in baking and cooking I would replace castor with granulated sugar

You can always substitute granulated sugar when castor is called for. It's a 1:1 exchange