Silver in this country is of sterling standard, which is 92.5% pure. Silver plating is done with as near as dammit 100% pure silver. As wheurhappy points out, there is a noticeable difference in the colour - the silver plated ones look more "glitzy".
This is all part of a depressing reduction in the quality of medals. The current issue of OBEs and MBEs which are made by a well-known firm of medallists which I will not mention are of very poor quality compared to the earlier versions which were either produced by The Royal Mint or Garrards, the crown jewellers. Even the boxes are cheap and nasty compared with the old plush lined cases.
And as for the 2002 QGJM - words fail me!!!!