Upgrade kits available, either slotted rotors for the original calipers, or Willwood calipers and vented rotors.
Scimitar SE5/5a Brakes L1 Archives - Scimitar Parts
The Salisbury axle looks like a Capri V6 item, so a rear disc conversion for a Capri might be an option? Or is it a Mk2 Jag?
Might also be worth joining https://scimitarweb.co.uk/