Plumbing – the correct tool to unscrew this mixer tap nut

It's called a tap connector (obvious now I've found it), and you should be able to unscrew it. You can see one here

It's the bottom nut you need to be turning. It's part of a joint that attaches the pipe to the tap.

Once you have this off you should be able to remove the tap as normal.

If it's stuck then something like WD40 or penetrating oil might help. Failing that carefully heating the nut with a heat gun might expand the various parts enough to loosen them. Take precautions to avoid getting direct heat onto the bath (it appears to be acrylic).

A more drastic approach would be to take a hacksaw to the thread to separate it that way.

Clean off mineral deposits with a brass or steel brush and a lime dissolver such as CLR or Limewaway. Towel dry as much as possible. Dry further with hair dryer in an attempt to evaporate as much moisture as possible; this, to leave room for penetrating oil to penetrate.

Aply penetrating oil such as PB Blaster or Liquid Wrench. Tap, tap, tap the the valve to set up vibrations to encourage the penetrant. A vibrator can be useful to assist, such as an electric tooth brush or electric engraving tool. Allow penetrant to sit for hours and tap some more after that.

Use a tight fitting wrench or possibly a pipe wrench to turn it anti-clockwise.