According to one discussion forum:
The club rented Green Street House and grounds in the Municipal Borough of East Ham from the Roman Catholic Church from around 1912. Green Street House was known locally as Boleyn Castle because of its imposing nature and an association with Anne Boleyn, who had either stayed at, or as some believe, owned the house. Hence renting the grounds of "Boleyn Castle" the name Boleyn Ground came into being. Today the ground is far more commonly known as Upton Park, after the area of London in which it is located.Anyway, back to the stamp sheet. The stamps are the Union Flag, with 10 different labels. The sheet is printed by ISP in litho, self-adhesive. In a limited edition of only 10,000 and designed by "Royal Mail and West Ham Design Teams', the folder was produced by Zentih Print and Packaging.
The Royal Mail product code is VA285 and the price is the usual £14.99. The first day of availability was 18 May 2016.
UPDATE 8 JUNE: I've now seen the sheet, and it's folder, and I'm disappointed that on this occasion there is no information about the images on the labels. Sure, we all know Bobby Moore, and die-hard fans will recognise all the recent players, and most of the black and white ones, but it wouldn't have taken Royal Mail and WHU much effort to have put this information either on the sheet or in the folder. In years to come, future owners of the sheets may have questions.