So I deleted that custom field and I'm using the default zone custom fields.
Here is what I've done for testing, which I hope is clear: 2 Countries Zones called United Kingdom and Scottish Highland. Scottish Highland has one state called Inverness-shire, UK has all the default states including Inverness-shire. I have then setup 2 Shipping Methods, one for the UK (Free) and one for Scottish Highlands (£17.50) and assigned them their respective zones.
So when I register an account and select Inverness-shire as my county, I am given the option to pick between the UK and Scottish Highland shipping method. The problem I've found though is if I try and add a new address (billing or shipping) it only gives me the option to pick Inverness-shire, which won't work if they need it to be delivered somewhere else.
I have attached a print screen for demonstration.
Any help or alternative suggestions would be appreciated. Essentially my issue is our courier service charges different amounts for UK delivery, Scottish Highlands delivery, Ireland delivery, Channel Islands delivery etc so I need 5/6 different shipping methods to these places and I can't seem to find a simple solution for this.