Most project owners will include UK shipping costs within the reward prices. However, if you want this to be an optional postage cost or have a choice of costs for international shipping, there is a way you can do this.
The Crowdfunder project, Catch the Sunrise, shows a great example of this. The project owner has added in a 'Questions' section within their project description...
There is a 'Pledge More' button on the payment screen when someone goes to pay for a reward, which allows the supporter to add extra on to the price of their reward.
Please note that you have to wait until you are live until you can see this payment screen to take the screen shot, so just add it in quickly once you go live.