Hello Gordon,
Welcome to Jaguar Owners.
I have found some information that may be useful.
This can be caused by a number of different issues. Most of the time this is caused by a bad brake switch. If the switch is shorting out, then it will cause the computer to think that the brakes are being applied, which turns off the cruise control. If the switch is fine, then it may have a bad speed sensor or the cruise control switch itself is bad.
One of the easiest tests I do is simply have someone drive behind me and see if the brake lights are coming on without touching the pedal. If it is, then you should replace the switch. If not, then further testing of the circuit has to be done
https://mechanicbase.com/troubleshooting/cruise-control-is-not-working