Firstly, all parts of the foxglove should be considered poisonous and not eaten.
As to the others, nettle root is edible, but the tops are usually tastier and less likely to have a diuretic effect on you
Dock roots are again edible, but do have high concentrations of chemicals like oxalic acid, which are not too good eaten in large quantities.
Wild carrot is definitlely edible, but incredibly difficult to distinguish from other umbellifers such as hemlock - avoid unless you're absolutely sure.
Sweet Cicely is another umbellifer with edible roots, but again this is only easy to locate when in season, and could again be confused with hemlock (although it smells and tastes strongly of aniseed).
As to other edible roots, your best bet is burdock - which is easy to identify, available all year round (although a bit tough in winter) and tastes really good!
Another edible root is silverweed, but this is not as common as burdock.
Ramsons (wild garlic) have rots that taste like garlic or spring onions, and can be used in the same way.
Pignuts are also very tasty, but these are more seasonal, and there's a knack to digging them up...
These are some of the ones I can recommend, but there are lots of others - however roots are often a more concentrated source of chemicals in plants, so always be absolutely sure you identify correctly!