Have your apprehensive husbands joined a site like britishexpats.com or british-expats.com? I know it can be a good resource for after the move - we found them for my husband during his first year in the US. But I imagine during the planning stages, it would be nice to troll around and see what actually British expats think about living in the US.
Assuming they are completely unfamiliar with expat boards, warn them that there will be people who love the US, hate the US but feel stuck there, hate the US and are in the process of moving back, or love the UK and are in the process of moving back. Not that I imagine I need to say this, but I strongly recommend you stay off the board and leave it as their private place to wander and vent as the move progresses. You know the kind of homesickness/hate-it-here things that are discussed on this board.