The £2 coins in base metal (as opposed to the gold double sovereign, which has a nominal face value of one pound too), nickel-brass, were introduced in 1986. They were modelled on the gold issues, having the same weight and diameter, although the base metal versions are thicker - since gold is heavier than base metal. It is not uncommon for people to think they have found a gold coin in their change; however, a true gold coin would be struck in a proof finish and would have a more coppery colour.
The entire series of seven different designs in five different years were all commemorative coins, produced largely to appeal to collectors. All are available in different metals versions (nickel-brass, silver and gold). Due to their low mintage, these coins are collected and kept by the public and do not circulate in any realistic manner.
There seems to have been an understanding that the gold versions of these coins represent a double sovereign (hence the same parameters), and they are referred to as such in some sources; this is also confirmed by the fact that the Royal Mint issued no St. George type double sovereigns in the years when these coins were issued.
The commemorative £2 are legal tender but contrary to popular belief this does not mean that banks and retailers automatically have to accept them.