How to change rear brakes with EPB, without special tools

You don't need to change the caliper, you need to adjust the brakes and the replace the brake pads..

Bulletin from Jaguar :

Electronic Park Brake (EPB)
Re-Calibration Procedure
2003 MY-ON
S-TYPE
M44998-ON

Issue:
On some 2003 MY-on S-TYPE vehicles, a customer may report a concern of premature
parking brake pad wear or the brakes binding. This condition can result if the electronic
park brake (EPB) system is not re-calibrated after the removal of the rear brake callipers
to replace the brake pads.
Action:
Should a customer express concern regarding the above, refer to the calibration
procedure detailed in this bulletin.

1. Reset the EB module:
a. Switch off the ignition.
b. Disconnect the battery for 30 seconds. For additional information, refer to STYPE
Workshop Manual Section 414-01.
c. Reconnect the battery.
2. Start the engine.
3. Firmly apply and release the foot brake five times.
4. Confirm that the message NOT CALIBRATED" or APPLY FOOT AND PARK
BRAKE" is displayed on the instrument cluster message center. This indicates that
the parking brake is in calibration mode.
5. Lightly press the foot brake pedal.
6. Apply the parking brake by using the EB switch.
7. Release the parking brake.
8. Release the foot brake.
9. Confirm that the brake warning lamp is no longer illuminated on the instrument
pack and that the NOT CALIBRATED" message is no longer displayed in the
message center.
10. Apply and release the EPB five times to ensure no error is present.
_____________________________________________________________________________

NB: It is also possible the actuator is bad though.

If it has the EPB (Electronic parking brake) it will only mean that the module is faulty and needs to be replaced, but if it is on manual handbrake than it can be few factors such as sticking cables, frozen pads, faulty cylinders.

Your A8 and new shape A6s with electric parking brakes require an electric input to the vehicle to tell it to wind back the pistons/pads. Main dealers have this on their diagnostic equipment. You can also purchase after market tools that just do this operation, heres an example
http://www.alpha-bid.com/product244.html
I have only ever used a main dealer machine so im not sure how good an after market one will be. Im not aware of anyway of doing the job without an electrical input to wind back the pads, the motors are very powerful so I wouldnt reccommend trying it manually. Hope this helps.