If you do not have rules based on postcode ranges, you can leave the postcode box empty.
If your shipping rules take into consideration the postcode, or part of the postcode, you need specify those in the postcode box.
In specifying the post codes, you can take advantage of the wildcard or asterisk (*) symbol. For example, if you want to include all post codes that begins with BS, you can then type BS*.
You can also specify post code ranges to save time in typing. If you want to include post code BS2, BS3, BS4, and BS5, instead of typing them individually, you can type BS2-5, and that will cover all those post codes.
By selecting United Kingdom, without entering any specific post code, that should cover everything in UK.
We have added this functionality to add more depth in creating your shipping rule. If you want to use a specific shipping method for a particular post code range, then you can use this feature of Shiptheory.