You can still run things off the battery with an isolation switch. Just have to run direct connections to the battery. Or to the live part of the switch.
As for my security, x defence flip up pedal lock, removable steering wheel, boss lock, ignition switch, main power switch, fuel line solenoid (200tdi engine so no fuel pump motor), alarm and immobilizer and lastly a tracker.
All wings and doors fitted using a mix of torx and hex.