Hi Andrew, I assume you have not yet undertaken a trial fit of your 18 inch space saver, at least to the front axle of your XKR? As Jim has stated above, the 18 inch, red wheel WILL NOT FIT the XKR as a consequence of the larger front brake callipers. It has to be the 19 inch space saver for the XKR, which in Jaguar supplied days was black in colour, whilst the 18 inch was red. Nowadays, there are multiple companies offering space saver spare wheels and one can no longer rely on selecting by the colour of the wheel. I have spotted black space savers that are only 18 inch.
Anyone who needs / wants to procure a space saver spare wheel needs to take due care and attention so as not to be caught out. And, DEFINATELY do not elect to purchase a suitable standard (front) road wheel as that would be dangerous if fitted to the car in a puncture situation. In these cases, it is not a diameter issue to fit over the rear callipers, but it is an "offset" issue to avoid binding on the spokes on the rear.
Look at a company called Road Hero, who use Kwik Fit as a retail agent. They offer a complete kit, including chocks and jack, which all came with the Jaguar supplied version.
N6 JMX