That isn’t an option currently. Out of interest, why would you want to do this?
As technically they are outside the United Kingdom and in my use case (prize competitions) it would be nice to have the option to exclude them for legal compliance.
Can you exclude them in the standard WooCommerce settings?
In WooCommerce -> Settings -> General options, you have “Selling location(s)”. If you set this to exclude specific countries, then you can add in Jersey, Guernsey and Isle of Man.
Although obviously this wouldn’t stop someone from just selecting UK as their country, which may of course be why you’ve posted this question!
Either way, I’m not sure adding such restrictions would be a good fit for my plugin — what you’re looking to do is less about stopping them performing a postcode lookup, and more about stopping them from checking out altogether, so I think a plugin that can do that would be a better option.
Unfortunately I don’t know of any off the top of my head, though.
Yes, I am excluding them in WooCommerce. I just thought it would be useful to stop the address lookup as well but it’s not really a problem.