From what I can find online, it looks like the XF uses the same bore and PCD as the disco sport, so you could physically mount a space saver to the hub, but the tyre is a different story.

It looks like the outer diameter for the XF tyres is between 26.5 and 27 inches, depending on whether it's on 18, 19 or 20 inch rims. The space saver will have a similar outer diameter (probably not an exact match, but close enough to be useable temporarily).

The DS runs tyres with an outer diameter of around 29 inches, which I'm guessing is just too much of a difference to make the XF space saver useable.

Best way to confirm this would be to check the tyre dimensions on the space saver, and the dimensions on the existing DS tyres (should be 235/55r19 for the 19-inch rims) and plug them both into Tiresize.com to see the size difference.

 

Is the XF space saver 19", or are the normal wheels 19" ?
My DS space saver is a 155/85 R18 tyre, so it's a 18" wheel. Fits my DS that came with 20" wheels.
Can find 17" XF steel space savers that will only fit XFs with 17" or 18" wheels.
Can also find 18" XF space savers with 155/85 R18 tyres, 135/80 R18, and 115/85 R18 tyres that do not specifically say that they will only replace a specific range of wheels.

In reality the brakes determine the size of the space saver that you can use. The D240 and petrols have larger front discs so need at least a 18" wheel, I believe that the D180 can get away with a 17" space saver - someone will come along and let me know if I am wrong.

IMHO the outside circumference of a space saver is immaterial, you will be limited to 50mph and cornering much slower than you would normally.
Gills Micra had a space saver that was much much smaller than the normal wheels, but it got her home which is the point.

 

In reality the brakes determine the size of the space saver that you can use. The D240 and petrols have larger front discs so need at least a 18" wheel, I believe that the D180 can get away with a 17" space saver - someone will come along and let me know if I am wrong.

IMHO the outside circumference of a space saver is immaterial, you will be limited to 50mph and cornering much slower than you would normally.
Gills Micra had a space saver that was much much smaller than the normal wheels, but it got her home which is the point.

Hi
You are right.
I’ve got 17’’ wheel for my d180. Photo in attachment.

 

I have a 155/85R18 spare with 235/55R19 road wheels, if that helps. Can't remember the size of either of my XF's spares but both were alloy - one was painted bright orange!