Plumbing – this old plumbing fixture and can it go

It looks like the screw in the center is missing (the hole in the center of the piece that says "off"). I would try pulling on that center part to remove it (the screw that was there probably was to hold it in place but now that it is gone it should come off... easier said than done). You will probably need to pull fairly hard or even use a screw driver to slowly pry it off (especially if it has been on for a long time it might take some work to get it off). Then once that center piece comes off you should see the other screws that are holding the rest of the unit on.

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.