Thank you for your comments. We get a lot of questions concerning whether other plants are safe around their foxgloves. The Lady Bird Johnson Wildflower Center, home of Mr. Smarty Plants, is committed to the growth, propagation and protection of plants native not only to North America but to the area in which those plants will be grown. In view of that and the fact that you are obviously a gardener in England, we believe we are talking about two different genera of plants.
From the Nova Scotia Museum The Poison Plant Patch discusses the true poisonous plant Digitalis purpurea. This plant is in the Scrophulariaceae family. There are 26 plants in our Native Plant Database with the common name "foxglove," frequently accompanied by the word "false." Sixteen of these are in the Agalinis genus, 7 in the Aureolaria genus and 3 in the Penstemon genus. All of these are also in the Scrophulariaceae family, but there are no members of the Digitalis (foxglove) genus in our Native Plant Database. The genus Penstemon has visual characteristics very similar to Digitalis. These native plants are Penstemon cobaea (Wild foxglove), Penstemon digitalis (Mississippi penstemon) and Penstemon fendleri (Fendler's penstemon). It would seem likely that some early botanist, possibly European, called attention to that similarity in naming Penstemon digitalis (Mississippi penstemon).
We are aware that many non-native and poisonous selections of Digitalis plants are sold in North America. They are beautiful flowers, evoking the traditional English garden, much admired in the United States. If they have only the common name on the labels and no scientific name, it would be very difficult for a lay person to avoid purchasing the plant. There is little Mr. Smarty Plants can do except remind everyone to investigate carefully every plant for sale BEFORE purchasing it.
One final note, your mentions of using Digitalis in growing vegetables is still out of our range of expertise. Most edible plants considered vegetables are either non-native to North America or so extensively hybridized that recognizing the origin and nativity of the plant is difficult to impossible.