Once the code is stable, brush it up with some re-factoring and showcase it (or present it) to the tester for user acceptance tests. If the tester comes up with bugs or with exceptions, go back to the previous step of writing more unit tests to be able to determine what code should be written for the user acceptance to happen!
In UADD, start backwards. Get the UAC and then start your TDD ! Simple !
BDD is just a step further ! :)
Happy TDD & UADD !