Magento - 未经身份验证的用户的客户组

In Magento I have set up 5 customer groups for pricing. The group table looks a bit like :-

Group ID | Group Name
1        | Northern Ireland
2        | Dublin
3        | Rest of Ireland
4        | Great Britain Zone 1
5        | Great Britain Zone 2

Each Customer that registers is placed into one of these groups depending on the County/State in their address.

Our products are priced depending on the delivery location so our product pricing would be a bit like:

Product 1
Group ID | Price
1        | £1.99
2        | £2.99
3        | £3.99
4        | £4.99
5        | £5.99

This all works fine for logged in users as they have already been placed into a group so they will be displayed the correct price for their location.

I want to give an unauthenticated user the option of choosing their state from a dropdown list, I can then pass a variable through to select the correct group pricing.

I have been going through the various files trying to find the location to place the code and am looking at app/code/core/Mage/Bundle/Model/Product/Price.php

There is an if statement here that I think may be the correct block to edit.

if (!$gId) {
    $gId = Mage::getSingleton('customer/session')->getCustomerGroupId();
} else if ($gId instanceof Mage_Customer_Model_Group) {
    $gId = $gId->getId();
}

Could anyone confirm this? I would be looking at adding another else if:

if (!$gId) {
    $gId = Mage::getSingleton('customer/session')->getCustomerGroupId();
} else if ($gId instanceof Mage_Customer_Model_Group) {
    $gId = $gId->getId();
} else if **Session Variable Exists** {
    $gId = Session Variable;
}

Thanks for your help, Liam