How do I compress the rear caliper to install the brake pads on a 2008 jaguar xj8

If the parking brake is built into the caliper, you must screw the piston back in. Many auto parts stores borrow or rent the tool kit to do that. Choose the tool insert with the correct tabs to engage the notches in the piston. As you turn the tool, it will screw the piston in and push it in at the same time.