If the coil is OK I would expect to see a low resistance but not zero indicating a short circuit. I assume you are seeing some resistance?
Do you have two fobs and this happening on both? I assume you would have checked this if you have.
substitution would be a good way to go but you could check what you see from the wires to the coil from the SCU. The SCU should output a voltage on one wire when the ignition is switched on with the immobiliser active and should switch it off once it is deactivated.
if you do not see this it could be a fault with the wiring or more likely a faulty SCU.
As you may know, if this is the Lucas system swapping the SCU will then require the engine controller be put into learn mode to learn the code of the new SCU and will require the fobs to be programmed to the controller. Both of these can be done with Pscan or a T4. If it’s the Pektron one you have to get new fobs too, or speak with Teknozen who should be able to help you
 

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