It's never said it would be £100The way the adverts talk about a smart meter saving you hundreds are nonsense. Sure it means more accurate bills but the only way it will affect someone’s bill is when they see ow much things cost. I doubt the change will be £100’s even over a year.
Originally the adver quoted ofgem saying average saving of £28 per year this then got altered to £11 per year and the wording changed to show you needed to watch your usage and understand how you spend money.
As a supplier we have hated the whole smart meter rollout to the domestic market, in general we earn less because people are re-educated as the masses haven't a clue that leaving things turned on costs them money! Honestly 90% of clients a re clueless that a 60w halogen bulb will cost x amount per hour to operate and the equivalent LED at 4w will cost x amount and save loads over its lifetime.