Hey have you looked at any trikes. The ICE Full Fat is made in England and has big 26" wheels for rolling-over rocks and ground clearance, and Shimano e-assist
https://www.icetrikes.co/products/full-fat 
Failing that look at some of the Dutch cargo trikes like Radkutsche Musketeir cargo trike. The Dutch are way ahead of the game on cargo trikes
