Looks like play money. There aren't a whole lot of true numismatic pieces made of plastic. Most are just play money. About the only other things are some Keeling Cocos issues, food stamp change tokens, or New Zealand practice money.
That was definitely one of the possibilities that I came up with... some sort of play money or token... the relief and quality of the design suggested to me that it might be worth checking out tho.. I know the US did some experiments with plastic planchets.
What size is it? Play money is usually either really tiny or really big to avoid becoming a choking hazard. It may also be a medallion of the coronation. There were thousands of different novelty pieces made in all different materials and quality.
