The 1894 Essex Directory gives the following details:

King John K. seed grower to H.M. the Queen & seedsman by royal warrant to H.R.H. the Prince of Wales; established over 100 years
The 1902 Essex Directory gives the following details:

King John K. & Sons, seed growers by Royal warrant to H.M. the King; Home & Export seed growers; established over 100 years. Telegrams, "Seedsman, Coggeshall"
And the 1914 Essex Directory gives the following:

King John K. & Sons, by appointment to His late Majesty King Edward VII; Home & Export seed growers; established over 100 years. Telegrams, "Seedsmen, Coggeshall;" TN 2
Presumably the royal warrant to the Prince of Wales automatically became the royal warrant to the King on Queen Victoria's death. However it didn't pass on to George V. This would date the medal to between 1901 and 1910.