Earliest Known Usage of Adhesive Stamp

Bedlington has a place in postal history as it has the earliest known recorded usage of an adhesive stamp to prepay postage. The Penny Black officially came into service on May, 6th, 1840. However someone sent a letter from London under a "Mulready" wrapper to a Mr. Blenkinsop of Bedlington four days earlier. This then made the letter the earliest known in the United Kingdom. Together with this, the Mulready wrapper was not officially valid until the same day as the Penny Black. The Mr. Blenkinsop the letter was sent to, was an engineer at the Bedlington Ironworks, and what makes the letter more intriguing is that he had died nine years earlier. The stamp and wrapper were worth up to half a million pounds in 1991. How much today? With Mr. Blenkinsop not being alive when the letter was sent, it was forwarded to his family who lived in Carlisle, bearing a May 4th, 1840 Morpeth postmark, probably adding more value to it.