The standard Royal Enfield Himalayan Spam tin panniers are OK, as a knock-off copy of the BMW panniers. If any of the securing nuts inside the panniers become overly tight, you’ll need a 16mm spanner (not included in the toolkit) to loosen them. I bought a cheap spanner on eBay, which is good enough for the job.
Note: The bike has a number of Allen bolts, including some on the pannier rails. For some bizarre reason, Royal Enfield decided not to include Allen keys in the toolkit. The sizes are: 4, 5 and 6. I think that covers them all.