In the past you could exchange Tesco Clubcard vouchers for Rewards Tokens and later get your points back if you changed your mind or simply couldn't use your Rewards Tokens for some reason.
However... not any more since they quietly changed the terms and conditions in August:
19. Unused, expired and redeemed rewards tokens issued after 13th August 2012 cannot be refunded, exchanged or returned. This does not affect your statutory rights.