Which kit did you actually go with?
Oh man, it's been almost 15 years since I did it with mine as part of a frame-off rebuild. It was one of the universal street-rod kits. Pick how many circuits you want... just tally up how many things you want control over to determine number of circuits. I added extra rear illumination lighting, interior lighting, fog lights for the front, electric fan, wired for a GM/Delco alternator, door light switches, rear wiper, rear window defogging element, electronic ignition, 12v auxiliary plug, second fuel tank electric switchover solenoid, rear tank gauge, rear heater blower... maybe more. It's all very robust and fully fuse protected. I stuck the fuse-box to the right of the steering wheel (LHD). There are a couple of more circuits still available. So maybe an 18-circuit kit non-keyed?
The only thing being an issue, if I recall, was the alternator connection. But I think any kit can be adapted for whatever you've got.
I had rebuilt my Rover to be more useful, not to be stock or "original". I would have gone with a more conventional replacement loom if I was concerned about originality. I think at the time, though, there weren't very many aftermarket "stock" solutions for reasonable $$$. The Painless Wiring looms were reasonably priced.
Of course, my Rover is the farthest thing from stock now... but the wiring is all good and still has extra capability if necessary.