Pikachu, What's This Key? available on Pokémon TV
This short originally aired alongside Pokémon the Movie: Diancie and the Cocoon of Destruction in Japan. It has been posted to Pokémon TV to celebrate the TV airing of the English dub of Diancie and the Cocoon of Destruction, which will air as part of a 4-hour hour marathon on Cartoon Network in the United States on Feb. 7, 2015.
The short is only posted to Pokémon TV on the US Pokémon site, not the UK Pokémon site.