Hi @WMSC Welcome to Shopify Community. Thank you for sharing your query here.
You can show a custom message in Cart page or a pop up that appears before someone move to checkout page. This message or pop up will have the information about taxes and Duty.
If you are not on Shopify Plus then this can be the best solution for your query.
I’ve managed to implement a message in the cart-drawer.liquid file using the {%- if localization.country.iso_code != ‘GB’ -%} condition which seems to work quite well.
I’d love to add the message in the actual checkout, somewhere around the shipping note on the right, but I’m guessing this isn’t possible?