The First Day Covers of Great Britain, generally referred to by collectors as “GB FDCs” is one of the most popular areas of GB philately. A First Day Cover is an envelope (or other type of enclosure device) bearing a stamp postmarked on its first official day of use. It is possible to collect first day stamp usages from the Penny Black onwards and Victorian and Edwardian stamps are pretty much all valuable, when they can be found at all. However it is interesting to note that postmarks specifically designed for the purposes of cancelling a stamp only on its first day by including the words “First Day of Issue” did not appear until 1963. If this does not seem quite right to you it will be because the issuance of new stamps often coincided with special events which had their own special postmarks used to cancel stamps, for example the 1924 & 1925 Wembley Exhibition sets, the 1929 UPU Congress set, 1935 Silver Jubilee & 1937 Coronation issues.
The 1890 Uniform Penny Post Jubilee envelope was probably the first souvenir first day cover produced in Great Britain and is known with special Jubilee cancellations, a 1911 illustrated post card bearing the 1d green postmarked on Coronation Day (and the first day of the stamp) can occasionally be found but the first stamp issue to be seen somewhat regularly on illustrated covers made to present stamps for cancelling on their first date of issue was for the 1935 Silver Jubilee set.
KEVIII and the reign of KGVI saw an explosion of illustrated FDCs made widely available to the public apart from some of the more obscure definitive values which were issued without fanfare. Today collectors of First Day Covers can add to their collections regularly and advanced collectors can seek out the scarcer versions of the different special postmarks available for most issues.
We would recommend the excellent “Collect GB First Day Covers” reference catalogue by Jeffrey Booth (“The Booth Catalogue”)