According to Wikipedia, "the British Post Office first issued self–adhesive stamps on October 19, 1993". A quiz on the BT website also gives 1993 as the year in which self–adhesive stamps were first issued (in Question 5).
But the (Daily) Telegraph reported on 9 November 2000 that "the Royal Mail is to sell self–adhesive stamps nationwide in the New Year ... From January 29 the new standard first– and second–class stamps will go on sale in shops and vending machines."An article in the Daily Mail reports that "Self–adhesive stamps were today on sale to the general public for the first time." I can't find a date in this article, but it goes on to say that "today's launch also commemorates the 100th anniversary of the death of Queen Victoria with a special "stamp label" in booklets." (Queen Victoria, as any quizzer should know, died in 1901. On 22 January 1901, to be precise.)
I'd love to be able to add something to the debate from my own recollections, but I'm afraid my memory is far from precise enough to give an opinion on whether I first bought an adhesive stamp in 1993 or 2001.
What appears to be the crucial clue is buried in the Daily Mail article: "Previously self–adhesive "sticky" stamps were only available, in rolls of 200 or more, to business customers."
ebay has plenty of stamps for sale that were issued in 1993 and are described as self–adhesive. They are not obviously exclusive to business users, and they don't appear to be in rolls of 200, so I'm not convinced that I've completely resolved the conundrum; but this is as far as I've managed to get.
© Haydn Thompson 2019