Hi Leftie!
Welcome to the Thorn Cycling Forums!
Having owned a Sherpa Mk2 (frame twin to the Raven Tour) and now a Nomad Mk2, I think the Nomad might be overkill for your needs. It is a first-class, top-notch Expedition Touring Bike capable of carrying enormous loads...but because of that, it is not as much fun to ride for general, all-'round and unladen/sporting use as a bike with a bit lower cargo capacity -- though I do ride mine all the time and enjoy it greatly in any use; it is just what I wanted and very much needed. The main difference is the frame's stiffness, which makes it more dependent on wider, fatter tires to gain back some of the comfort of a more resilient frame when riding unladen or with smaller loads.
I do think the Rohloff drivetrain would be ideal for an urban works bike, especially given the lower maintenance requirements of the Rohloff hub.
I think of the two, I'd go for the Raven Tour to keep a livelier, in-town ride. While 10kg in a backpack is a lot, 30kg in F/R panniers isn't that much for a RavenTour (or the new ThornRaven, that replaces both the Raven Tour and Raven Sport Tour). If you consistently carried that much or more, then I would definitely suggest the Nomad, as the (New) Thorn Raven's maximum capacities range from 33-40kg depending on frame size (tube diameters are proportional to frame size).
Have you given thought to a small cargo trailer, perhaps something along the lines of a BOB with a large platform that could take your present pack and perhaps additional supplies as needed? Either bike would tow the trailer nicely, carry the load without a complaint, yet be light(er) and sprightly when the trailer was left behind.
In any case, I assume the bike will be parked outside while you're fumigating indoors. With that in mind, I would give considerable thought to good security. Hub security (in the form of Pitlocks or Atomic22 security skewers), a good U-lock and heavy chain, and perhaps a ring-lock wouldn't be amiss in a city-core setting like Central London. The locale -- and leaving the bike outside -- is what makes me wonder if either of the Thorns might be unduly theft-attractive for your needs. I think if I went for a Rohloff Thorn in that setting, I'd try to keep the build pretty basic and go for the stealth black and let dirt accumulate a bit to make it less flashy to thieves' eyes.
I hope this helps. Others will surely be along shortly with good suggestions of their own. That's one of the beauties of this Forum; you'll get a crowd-sourced answer to nearly any question!
Best,
Dan.