I've recently bought a 1998 P38 Range Rover, I'm planning on fitting some LED bulbs, with the interior bulbs will they cause an error to be displayed due to the wrong resistance or is that only with the exterior bulbs?

You`re right that a fault will be displayed if LEDs fitted to exterior lights. I imagine that with interior lights it`s not a failure/warnihg system as with interior lights you don`t need remote warnings to show the light ain`t working, you can see the light ain`t working! Pull a bulb and see if fault comes up, I bet not. Try it and let us know!