Here is another example from a UK suermarket https://groceries.morrisons.com/webshop/product/Nutmeg-Nappi... which is 0.075 GBP (0.10 USD)/diaper.
The UK doesn't charge sales tax on nappies/diapers - https://www.gov.uk/government/publications/vat-notice-714-ze... which may explain a small part of the difference.