In most shops, there is a "tall" section, go here and buy what is meant to be a top...and wear as a dress!
Tops are always cheaper than dresses! I found out this tip today when rummaging around New Look's sale (which has been on a while) and found a perfect dress for work. Tried it on - perfect length! It's technically a top for tall people, but comes to just above my knee.
And, it was only £6. This is a good tip if you feel an item is too short for you, go over to the tall section and they are bound to have it. Just a little bit longer!