BT are right, they cant guarantee wifi will work in your location as they have no control over your neighbours use of whatever devices they wish to use, all they can do is move you to other channels to see if that works, but if someone has a device they use here and there, that is on an empty channel you use, they will probably interfere.
I have a video sender that interferes with my wifi reception, luckily the video sender has 4 channels so i just pick one that doesnt clash with my wifi, however thats not to say that the channel i picked isnt causing problems for my surrounding neighbours, all it takes in one or two video senders and 8 or 9 wifi connected devices, and it would be a nightmare.