Most grocery stores throughout the US carry a few British products, not always in their own section. Digestives (McVities especially) are popular. Some of the sauces and teas. Typically not the candy, biscuits, and crisps though :(.

Most places with say at least a couple hundred thousand people will have a dedicated British import store, usually run by an expat with a much larger selection.

I think watching too many BBC dramas (Netflix you aren't fooling anyone with your "Netflix Original" British shows) causes many Americans to seek out some of your more interesting products. And we do actually like tea, its not as popular as coffee but we drink a fair amount of it.

Also the Bake Off has caused a huge surge in baking here and while most stuff is easy to get you definitely use a few items that are hard to substitute here. I was ecstatic to find Treacle and Golden Syrup the other day in Nebraska. Definitely some delicious bakes soon. I just wish we could get your double cream.