[DrJokepu]

That's not quite true - all they buy from BT is the "last mile" between the telephone exchange and the customer end point. Everything else is on the ISP's own (or rented) infrastructure.

[loikujhygthj]

True, all they rely on BT for is the bit that's completely impossible for them to get anywhere else or duplicate.

[electromagnetic]

Actually they're completely free to create their own last-mile connections, most simply choose not to and try to irk out their living being the middle man.