I've now spoken to the manager at Thrifty Karratha, who referred the issue onto a member of Thrifty customer service in Sydney. The problem is being resolved by refunding the $95 to my credit card, and removing the reference to unlimited mileage from prepaid.thrifty.co.uk.
Both of these people have been great to deal with, it's just a shame about the way it was handled initially by the front-line staff at Karratha.