Can I have white potatoes if I’m not bothered or allergic to nightshades? Sweet potatoes (which I dislike) aren't allowed on SCD but are allowed here, so why not white potatoes?

Yes and yes, you can have white potatoes in moderation (as long as you do not have raging candida overgrowth, in which case 1/2 potato is your max at one sitting). Listen to your body. White potatoes are higher on the glycemic index, so blunt the sugar spike with some butter, ghee, or coconut oil on top.

Sweet potatoes are not SCD legal, because they are high in soluble fiber which acts as a prebiotic (food for good bacteria), and those with SIBO or a bacterial overgrowth get bloating and pain when fiber is introduced prematurely before the bacteria has been knocked down. Your body will guide you. If you’ve followed SCD for a while, you can start trying new foods after the transition week to see if you can tolerate fiber.