I'll take a crack at it. Assuming you have the current UC installed (Flatrate works differently in pre-Beta):
- Enable Flatrate module
- Navigate to admin/store/settings/quotes/methods/flatrate, click "Add a new shipping method"
- Fill out the four boxes:
- "Shipping method title" is your internal name which shows up in the admin menus. For example, "Shipping to UK".
- "Line item label" is how the shipping method will show up on the checkout page.
- "Base price" is minimum shipping charge for this method.
- "Default product shipping rate" is additional charge per product.
- Use the Submit button to commit these settings
- Now go to admin/store/settings/quotes/methods and check the box next to your newly created shipping method.
By default, this shipping option will show up for all orders. The trick now is to configure Workflow-ng so that it only gets applied to certain orders - in this example only orders sent to the UK.
- In the menu at admin/build/workflow-ng, click on the name of the shipping method you just set up. "Shipping quote via Shipping to UK" in our example.
- Click on "Add a Condition"
- Choose the option "Check an order's delivery country" and press the "Add" button.
- From the selection of installed countries, choose "United Kingdom" and press "Add". (I don't know if you can select more than one country here, but you can certainly add multiple conditions, one for each country, then "OR" them together.)
And with that, you have made it so your custom Flatrate is applied only to orders sent to the UK.
Now you have to repeat the entire procedure above to create a shipping rate for the "Rest of Europe", and another one for "Worldwide". "Rest of Europe" will probably consist of multiple "Check an order's delivery country" conditions ORed together, while "Worldwide will consist of a condition where the delivery country is logical NOT one of the selected countries. You now have three shipping options enabled and active, but only one is in effect for any destination country.