There is no such thing as "Customer Loyalty". This applies both ways. There is only a customer. Either a customer who moves annually because of the price offered to a new customer by another ISP or the customer through inertia that does not bother to move regardless of the price.
From BT's and other ISP's point of view they know that the vast majority of customers do not move because of the price or a price rise and for every one that does move there is a new one coming along because of the incentive offered by the ISP to new customers.
If that was not the case you can bet that the bean counters would ensure that the incentive to stay would match if not exceed the incentive to become a new customer.
You would appear to be in the latter camp and do not move regardless of the the price. If you are doing this out of "loyalty" to BT perhaps you should look at what you mean and expect from this "loyalty" because as far as I can see it means nothing to BT or any of the other ISPs.
This also applies to Insurance, Energy and many other companies. You owe them nothing as regards "loyalty". All they want is your money.
