How can I block certain postcodes before order placement?

[cybko]

Correct me if I am wrong, but am I correct in thinking there's no way to stop an order before payment currently?

Essentially I need to block certain postcodes. Shopify doesn't understand UK postcodes so I can't do it with the delivery profiles.

I can't see how to do it with Shopify Flow until the order is placed.

Is there anything I'm missing? I have no desire to add more apps since Shopify won't let me use single origin shipping