It is an offer not a guaranteed price. It is to attract new customers not to let current customers reduce their rate. You can cancel your account and sign up for a new account but is it really worth it? If you bought a car and three weeks later the price was dropped by £3k, would you demand a refund? What do you do when Waitrose have a special offer that you bought two weeks previously for more? When you signed up with the ISP you were obviously pleased with the price and considered it to be a good deal.