OEM setups typically have the kickdown switch somewhere on the throttle and requiring an extra push at full throttle to engage – why it’s called “kick down”. Hence the driver has the option of applying full throttle without kickdown, if that’s what he chooses to do.
On the V12 XJ-S, though, the kickdown switch rarely works. For one thing, the pedal itself is too weak, so when you push it to try to get kickdown it just flexes rather than engaging the switch. After enough time it even takes a set, so it couldn’t engage kickdown on a good day. That all can be corrected, of course, but it’s of interest to note that some owners prefer to do away with the manufacturer’s intention here and instead install a microswitch on the throttle that engages kickdown without requiring the extra push, just roll the throttle on and it kicks down. The guys who do this often claim they vastly prefer how the car drives when it’s so quick and easy to get it to kick down. I suppose some who bought the car for its luxurious and serene ride might not care for all the downshifting.
You definitely want kickdown to be available. Whether or not you want it to require a decision or to just come on whenever you’ve opened the throttle is something only you can answer. Either one shouldn’t be too difficult to rig up. I do wonder why yours seems to be missing.