on the checkout page add a the next day shipping checkbox with a checked value of 1:
<input name="NextDay" type="checkbox" id="NextDay" value="1" />
on the confirm page, near the bottom, is a collection of hidden form elements. Add another one with the same name as the checkbox to capture the value from the checkout page:
<input name="NextDay" type="hidden" id="NextDay" value="<?php echo((isset($_POST["NextDay"]))?$_POST["NextDay"]:"") ?>" /
then add a set session value behavior, open the server behaviors panel and click the plus button, select eCart -> General -> Set Session Value.
For the trigger and the value, click the lightning bolt icon and select the NextDay form element. Set the name to NextDay.
now you can set the shipping rules on the shipping tab.
For the Trigger use based on session Variable Value:
if the session variable "NextDay" is "=" a value of "1"
to make the rule specific to the UK, add another trigger:
Based on session variable value:
if the session variable "eCartCheckoutForm_shipping_country" is "=" a value of "UK"
and set the calcualtion to a flat rat charge of 10
you can create other rules for other countries using the eCartCheckoutForm_shipping_country session variable