Newspapers are normally sent to retailers on an 'SOR' (sale or return) basis, meaning that the retailer is charged the wholesale price of each newspaper but that amount is then refunded if the paper is unsold and returned to the wholesaler.
So Tesco will have suffered a financial loss through your action. (Yes, I know it's only a few pence, but it's still a loss). A criminal prosecution for theft would almost certainly fail because it's a statutory defence to show that you genuinely believed that the owner of the property would not object to you taking it..
However it's still sufficient to count as 'gross misconduct', which can provide sufficient grounds for dismissal. There was a case, before an employment tribunal, when a Woolworth's employee picked up a 'pick and mix' sweet, which had fallen onto the conveyor belt, and popped it into her mouth. The sweet could not have been sold (for food hygiene reasons) but Woolworth's management still regarded her actions as 'gross misconduct' and dismissed her. The employment tribunal upheld that decision.
You'll have to hope that Tesco's personnel staff accept that you took the newspaper as the result of a misunderstanding. If they decide to sack you you'll almost certainly be unable to successfully challenge their decision.
Chris