If your UK based, and your visitors will be, you can use XMLRPC to query the Royal Mail database to return postal code details. Then using a similar interface with multimap.co.uk you can return the distance by road between users.
Tiki Maps is based on line of sight. its quite easy to add the postal code to long/lat referencing directly in from the royal mail db or even multimap.
Let me know if you want help there. Id be interested in getting a Tiki Maps project to work on.