Apparently woad was grown in the Fens (near Spalding, Lincs., I think) until the 20th century. The dye was made by fermenting the leaves of the plant. Eventually the trade died (gedditt? Dyed/died - oh well, please yourselves! Actually, it's not funny at all - don't know why I started this!) as a result of cheap imports of indigo ... from India (oh noooo - that's not funny either!!!).
Anyway, along with some of the previous posters to this thread, I grew some woad plants (Isatis tinctoria) in the garden - but I never did manage to extract any blue dye from them.