I've got one of those! Sylvster's right, its a diamond jubilee medallion. They were origionally struck as matte proofs (had a frosty appearance). They also exist in gold and bronze. Value wise, its a bit sketchy, I've seen them for sale from £4 to £100 in Uncirculated (no wear or damage). They would have also come in a red presentation box, which will increase their value.