It's all very well companies asking their customers to pay by DD, but to force such a payment style is wrong. There are numerous examples of payments being incorrectly deducted from numerous suppliers. If people want to retain control of their money, that should be their choice. And yes there is the so called Direct Debit Guarantee, but have you ever tried to enforce it
The major question though is whether the companies using such "fees" are trying it on or not. Depsite when the contract may have been started, most do carry terms that allow variation with notice. Now unless that variation is, in law, unfair to the consumer 9shows bias to the company), if notice is given it's reasonable. From what I can see, given that there has to be some manual billing input for any payment that isn't automated such as Direct Debit, it would be difficult for any customer to prove that BT's £4.50 a quarter is anything more than a service fee, and thus reasonable, equally the onus is probably on BT, who raise the fee, to prove it's reasonable.