A session variabel is already created that can be used for this.
In the eCart object you will need to create two shipping charges, one for UK orders, one for international orders:
UK Orders:
Triggers:
Session Variable Exists -
Session Variable name: [eCartCheckoutForm_shipping_country]
AND
Based on Session Variable Value -
If the value of session variable [eCartCheckoutForm_shipping_country] is [=] a value of [UK]
Calculation:
Flat Rate charge of 3.75
----
International Orders:
Triggers:
Session Variable Exists -
Session Variable name: [eCartCheckoutForm_shipping_country]
AND
Based on Session Variable Value -
If the value of session variable [eCartCheckoutForm_shipping_country] is [!=] a value of [UK]
Calculation:
Flat Rate charge of 5.75