Gokelstone,
afaik, FSB customers were, and are, able to choose just whom they want as telco provider. Most will be BT I suppose, and someone else for mobile, and someone else for broadband, and...
The IDN package is white-labelled as "FSB Telecom", so subscribers will get not only the benefits of the IDN services (including single billing), but also the feeling that they are under their organisation's umbrella.
They won't be told -unless in small print somewhere on the contract- that the service is provided by IDN.
In fact, the link stresses that it's our (ie. FSB's) service.