Replacing rack, wonder how to drop the sub frame safely

 

The task is a large undertaking but is doable. The first thing to do is go to the link listed below and scroll down to the sections that have the Jaguar Spec books heading. Then download the section for ( X Type Chassis Specifications) starting about page 418 you will find all of the info for changing out the steering and how to lower the front sub-frame.
You will get the general idea and then have to improvise the tools needed for lowering it.
Best of luck to you. Read up on it and then come back if you have more questions.

JagRepair.com - Jaguar Repair Information Resource

 

I haven't done this on a Jaguar, but I am pretty sure one way to support the engine from above is an engine cross support from Harbor Freight. Or you may be able rent one.

Joe