A quick update to my situation - rear braces replaced this morning, the work only took a few hours at my dealer of choice, they also serviced the rear diff at the same time.
In summary, I am happy with the outcome. However having rust free rear x-braces does highlight just how horrible looking the rest of the underside looks.
Whilst it is all cosmetic, it's shocking that Jaguar think it's okay for a prestige vehicle to look like this in just 4 years. The budget S.Korean manufacturers don't even have this problem.
Edit: Pictures now included
https://drive.google.com/open?id=17La6O7yvMcRsKJLfsRaS39aLSBDon5ec
https://drive.google.com/open?id=1reSPBs8XmSSHwzwoKILoT6j7D-mX5oMd
https://drive.google.com/open?id=1bbgVU3PrUe3PVxbma8mfo4D6cvwVlQiq