Good news, this means the master socket downstairs is wired and functioning as a master socket (demarcation point). So, simply disconnect the WIRES from the FRONT PLATE, then screw it back together and you can remove the upstairs socket and the other sockets if you wish. This will also remove the backwiring issue. No need for a professional.
Thank you Cajar - so I disconnect ALL the wires in the working master socket, not just one particular one? Should I then just push the now free wires back into the master socket and re-screw?
With regards to the bedrooom socket - I'm assuming cutting this off won't affect my cable internet? The modem is connected to the Telewest box (that you can partly see in the first photo) via coaxial cable. I'm assuming telephone is standalone from cable?
(I'm only asking because of the proximity of the two).