The only way to restrict this, is by using shipping.
Select 'Total Value'. In the area that says 'Excess' choose the option 'Take highest value in table'.
Select all of the zones, except for the UK, in the 'Shipping Bands' area, right click and delete one by one.
In the UK shipping band zone, highlight the classes and delete them one by one, until you have one remaining.
In the table on the right in the column headed 'Order Value' enter 0.01 and in the 'Cost' column enter 0.01.
Then tick the box that says 'Free over' and enter 0.01 in the field.
Ok that box and upload the changes.
This will now display United Kingdom as the shipping destination with free shipping, it will not allow the 'Country' field in the address to be changed to anything else.
Select 'Total Value'. In the area that says 'Excess' choose the option 'Take highest value in table'.
Select all of the zones, except for the UK, in the 'Shipping Bands' area, right click and delete one by one.
In the UK shipping band zone, highlight the classes and delete them one by one, until you have one remaining.
In the table on the right in the column headed 'Order Value' enter 0.01 and in the 'Cost' column enter 0.01.
Then tick the box that says 'Free over' and enter 0.01 in the field.
Ok that box and upload the changes.
This will now display United Kingdom as the shipping destination with free shipping, it will not allow the 'Country' field in the address to be changed to anything else.