Hey @TheTechSurgeon, welcome to the Square Seller Community! This is a great question. 😊
At the moment, there isn't a way to override a discount rule if the quantity amount is being increased. It's a great feature request, so ill make sure to show our product team this post for further visibility.
As a workaround, you could create the Buy One, Get One (BOGO) discount and apply it to the 3 items for £10, and then add the additional two items and then manually add a discount for the remaining difference during the checkout process. Just note, a manually-added automatic discount will apply to all items in the cart, not just items selected in that discount’s rules.
Hope that helps!
