Maybe that is the problem. I would guess that the screwed end is there to make a fluid-tight joint ??
When you first posted pics. I could not work out why the U-shaped slot was there.
Would not be too difficult to remove the tube on a lathe, strapped to the cross-slide?
Sez he, not having seen it ...

Do you have a drawing of it? Is the tube supposed to be just a tightish fit in that cast rib?
Is it gobbed up with some sealant to stop any fluid going up between the tube and it's hole? ie Can it be driven out? A blast of heat often weakens any sealant.
EDIT
Crap, my brains stopped again .... remembered ..
Is it possible to tap the inside of the stuck tube, and draw it out with a 'studding/ washer/nut/tube' puller
Dave
As 1/8" BSP is .383" nominal, I bet a 3/8" drill will be OK. Maybe the bit of tube wall left would collapse?
Baffled .. again .. just getting over a lousy head cold ...

Dave BC