Magento Store - Change Store View control form URL to Sessions

We have a Magento v1.4.0.1 installation with three stores, for the US, UK and EU. Currently we use an GeoIP lookup with a redirect to direct our customers to their region’s store. So a UK customer would go to [login to view URL] but get redirected to the UK site at [login to view URL] While this system works it leads to duplicate content in Google. To avoid this we want to change from the setup described above to one where the store the customer is on is handled via Magento sessions, rather than the URL (so a UK customer remains on [login to view URL] but sees UK store view). The customer needs to be able to change the store view via an option in the header. All external links containing store URLs need to direct customers to the correct page and session after the change. This needs to be done without damaging our Google SEO position. Work will be conducted on our development site before being transferred to our live site after testing.