A broadband cease involves a BT engineer attending the exchange and physically disconnecting the broadband side of the line from the distribution frame for which they charge the ISP £25 which all ISPs then pass on.
There is a difference between any expenses, like this, an ISP may incur and what the customer is contracted to pay for.
Customer has no contract with BT.
In other words, regardless of what an ISP pays out in servicing its customers, it cannot pass on that expense unless it is covered in the T&Cs.
Having said that, altho' the OP says it was not in T&Cs at time of taking contract, the T&Cs almost certainly have a clause that changes to them can be made at any time by ISP upon notice and are taken to be accepted unless rejected within a given period.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC