While we don't have a way to apply a discount based on country, you CAN create a specific Shipping Rule for the UK when Min. Item Total is 15.00 or more. Go to Seller Admin > Cart Shipping Settings, then click Next to proceed to the Shipping Rules screen and add this new Shipping Rule:
Shipping Rate Calculator: Flat
Shipping Amount: 0.00
Country: United Kingdom
Min. Item Total: 15.00
Click Submit to save the Rule.
Click Back to Admin, and you're done!