If you really want to do this, then cut the 20 mm strip off the whole length of the wall side of the bath and most of the chipped enamel can then be covered with tiles
I'd forget about cutting the bath, for the reasons given. Either get a narrower bath or remove the door and trim half the necessary from each side. I'd go for the latter unless the door is already a bit narrow (bathroom doors sometimes are). It will still look as much a period door.
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