'However, with eBay Simple Delivery, the buyer is forced to pay nearly double that (around £3) for no added benefit.'

In one word, tracking. SD includes insurance against loss and damage. How can they pay out for loss if the item is not tracked? they would be inundated with false claims.

What you should be concerned about is why your prices are inflated by a buyer protection fee because you have certified that you are a private seller.