Sounds like you're almost there. One of our client is looking for this functionality and is shocked it's not part of Ubercart already. I tried to explain that it might be better to just leave things as they are but he's scared of being ripped off. Also, he needs to be able to only allow UK Mainland customers to checkout and all others to complete the order process without making payment. I guess this is an important step towards that otherwise the system would not know where the customer is based before checkout.

Do you have an idea when you'll get chance to look into the hooks? Do you need any help from me?