Literal blood and sweat (but not quite tears) later - the additional tube is off the copper pipe. I rang the shower company, those teeth were in there to stay - they told me the only way to get it off was to cut it.
I won’t go into exactly what it involved, but it involved me standing on a suitcase in the shower (too short to reach), with a borrowed saw followed by nail scissors.
The original toothed tube will be staying on there until the house gets knocked down, or I move, whichever comes first.
The next step would be making holes in the tiles to attach the new shower piece to the wall, but that will be done by anyone but me.
Thank you everyone for your help today. You’ve been invaluable. If you ever see me on here again, tell me to leave 😂