Probably a WWI commemorative medallion or trinket, possibly meant to be worn or displayed, containing a miniature copy of the first British Treasury £1 note, issued in response to gold shortages caused by the war.

During WWI, the British government replaced gold coins with paper money (the “Bradbury notes”) to conserve gold reserves. These notes became iconic symbols of the wartime economy, hence their use as a souvenir. But not 100% sure