You appear to be thinking that turning off BT WiFi will turn off your Internet access. That isn't the case. BT/EE WiFi has nothing to do with your main WiFi, it is an additional service that will have no impact at all on your Full Fibre connection. As @Crimliar said, if you opt out it simply means that you lose access to hotspots while out & about & as @licquorice said, there are no costs or savings involved.
It nw seems that your original post about phone service was in fact a mobile phone. There is no connection whatsoever between your home broadband & your mobile phone.
I can only assume that you use a virtual network operator that uses EE to provide service. EE haven't invaded your phone & taken control of it.
As for “I am using the home access method the BT instructions told me to use.", I have no idea what that means.
@counterproductivewrote:
Clearly the advantages and disadvantages of EE wifi hotspots remain unanswered...
No, @Crimliar explained that to you.