Pret are doing a promotion to get people to use their Pret cards at the moment.
The Pret Card is a card you charge up with credit and then use to pay for your coffee with - like a gift card, basically.
They're giving £2.10 credit to anyone who registers a new Pret Card at the moment. Just pick one up in one of their cafes, then follow the instructions to register online. You do have to give your name and address, but you don't have to add any cash onto the card to begin with.
Once you've registered, your card will automatically be credited with £2.10. You can spend this on a free cup coffee straight away.
I did this on Saturday and it worked fine.