HI Pennyblackie. This is my point exactly. Most internal UK stampless letters sell regularly on
ebay in the range of 1 pound to perhaps as much as 3 pounds. Most UK stampless letters to mainland Europe countries sell in the range of 3 pounds to maybe 8 pounds. Trans-Atlantic ones to the US commonly sell in the range of 5 pounds to 15 pounds. Of course scarce markings will increase these prices, but these very modest prices are for the great majority of UK stampless covers. Other European countries stampless covers tend to be in the same general price range. Compare these prices to those of covers you mention with penny blacks, for example. I think that the stampless letter market is underpriced, but this is what they sell for and there seems to be no shortage of sellers at these prices so I don't see the values going up anytime soon, except perhaps for the real rarities or covers from very famous people.