Platter- Hallmarks question

[Kincher]

I found this platter the other day and am very interested in the hallmarks as I cannot find any information on the fleur des lies; I assume the 621 refers to the quatity of silver used, the crown we assumed was sheffield, however we read in another post that without the lion it did not originate in Britain, and so we are lost and confused any help would be much appreciated, thank you in advance.