The free WiFi that BT offers is basically a small amount of your given bandwidth is allocated to anyone who uses/has access to BTs free WiFi, you also have access to other people’s free WiFi when you are out and about.
The OP was asking how to turn off this section of the WiFi and mentioned it was interfering with his network but in reality yes it does create 2 channels on the same as your primary channels but interference would really only be an issue if someone was using it and that those channels are on the same channels as his other networks.
This is why with mesh you should try to not have the same channels overlapping but in reality many users with BT equipment need not worry or even notice a drop in performance.
So the OP either moves to a third party router that will not broadcast the free WiFi or they opt out of this insuring that those channels are off and not simply hidden.
It could be they just don’t want that signal broadcasted as I doubt it will be an issue with performance.
"You can't have your cake and eat it. Either you provide a BT WiFi signal in order to be able to make use of the BT WiFi network or you don't provide a signal and forego access. "
Unless you don't use the BT router, of course.
The next question is, how have you confirmed its the routers Wifi thats causing problems with the disks ?
Worth mentioning that its normally not wifi on the same channel that causes problems (because the AP's can then see each other and deal with collisions) but the overlapping channels that cause problems as they essentially become interference.