You keep quoting precedents but until someone had gone to court about this particular issue then the precedent hasn't been set.
As wingco1 correctly says, case law applies by analogy. The law could not operate if every circumstance required an individual ruling.
The aim of the legal system is to be consistent by deciding a matter of law in the same way as previous decisions. This does not always happen in practice, but the system of precedent mostly works well.
Consistency is mostly achieved through the operation of binding precedent, which can only be departed from if there is something amongst the facts that distinguishes the matter being decided from the matter behind the precedent.
Cases that are not strictly binding are usually regarded as persuasive authority on the matter decided. Judges will usually attempt to honour persuasive authority unless there is good reason to depart from it. Relatively few matters require true judicial innovation.
Check any BT phone book.
It states that BT own the line up to and including the master socket. After that it is the owners responsibility. There will be a charge if an engineer is called and a problem is found to be with the wiring after the Master socket
This raises the question of whether a BT Phone Book can speak to the terms and conditions of a Plusnet contract. I believe it is BT Retail who produces the book these days.
Unless a party has a reasonable opportunity to acquaint themselves with the terms of a contract before entering into that contract, then, barring a few narrow exceptions, they are not bound by any alleged terms that they subsequently encounter. The scenario behind a key piece of case law here was whether a hotel guest was bound by the terms in a notice in their room limiting liability for lost or stolen items. The court held the guest was not bound, as the contract was formed no later than checking in, at which time the guest had no idea about the exclusion clause in the notice.
If a Plusnet contract says that you are bound by terms printed in the BT Phone Book and you are given an opportunity to read the current version of those terms before entering into the contract, you may well be bound by them. In all other scenarios, it is doubtful the Phone Book speaks in any way to the terms of a Plusnet contract.