I also have bags in the car that seem not to be with me when I reach the checkout. Before hypermarkets folk walked to a local shop for just the day's shopping; seemed more difficult to forget a bag those days.
If it's been such a success so far why change things ? Another 10p isn't going to change folk's behaviour and reveals it's purely for extracting more money. If they were serious about the environment then they'd supply non problem bags/boxes instead, but there's less cash extraction from the public that way.