They are actually legal tender. This is part of the New Zealand tourism office's "100 percent Middle Earth" campaign.
In the 2006 Christmas Special, "The Runaway Bride," the doctor created a diversion by causing an ATM to spit out money. The prop money used in the show were 10-pound notes with David Tennant on them and 20-pound notes with producer Phil Collinson on them. Now the BBC is letting you print out your own.