Yes, it is the one in the hall. I am kind of hoping it's not the master, as it's in a very awkward location (the nearest power socket is about 10 meters away).
How can I disconnect extensions, though? any guide you can link me to?
First I would carefully disconnect the wires from the suspected extension sockets noting the positions of the wires and not allowing them to touch each other. After each disconnection check that there is a dial tone at the suspected hall "master", if there is a dial tone then a "master" is established. Then I would disconnect the extension wires from the "master" thus leaving only the feed from the BT80 connected.
If you don't have confidence in doing the job then it's probably best to get an Openreach engineer, or other authorised person to fit a NTE5 master socket + filtered faceplate in a more convenient location. Or perhaps retain the original "master's" location and replace it with a NTE5 + filtered faceplate and use a "data extension" from the NTE5 filtered faceplate to a more convenient location thus allowing filtered voice extensions to be reconnected (less bell wires) at the original "masters" position.
I don't recommend that you personally fit a NTE5 master socket as a replacement since that is not permissible but if you can remove the extensions from the suspected hall "master" that would be a good thing in order to hopefully prevent high error rates once the VDSL2 is activated.