I'm also in the UK and I agree that it could be made clearer - I've seen a number of customers (and designers) getting confused by this layout.
However, it isn't possible to change the way it is shown. You cannot change the layout, remove this line or change the label. Also, you cannot use code to change this either, because the checkout page is "locked" to keep payments secure. The only other pricing option available is to show prices without VAT and then add the VAT to the subtotal.
I appreciate this is not what you wanted to hear, but I hope this saves you lots of time looking for a solution that doesn't exist 🙂
To make a feature request and ensure it is seen by the right people at Squarespace, you can do so via Squarespace Customer Care using a support ticket.
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