The page on the official site says that Zygarde will only be available for one week, but that page itself and the press release confirm it will be available until May 26. This is likely an error transferred from the US page, since the American region distribution only runs for one week.
The Zygarde distribution has actually been available since April 29, despite being intended to start on May 2, 2016.
“London, United Kingdom—3 May 2016— Celebrating the Pokémon Trading Card Game debut of Zygarde, owners of Pokémon Omega Ruby, Pokémon Alpha Sapphire, Pokémon X or Pokémon Y can grab the Legendary Pokémon now via Nintendo Network for a limited time.
XY—Fates Collide is the latest expansion to the Pokémon TCG franchise, featuring the powerful BREAK Evolution mechanic. Mega Alakazam-EX sees the future—and shapes it to unify two worlds! The Legendary Pokémon Zygarde arrives in many different Formes to bring order, together with Lugia BREAK, Mega Altaria-EX, and Umbreon-EX. Fates are sealed, and two Pokémon worlds join together! Booster packs and two theme decks—Sky Guardian and Battle Ruler—are available in stores tomorrow, introducing more than 120 new cards to collect and play.
XY—Fates Collide marks the first time Zygarde has been available in the Pokémon TCG. To celebrate this, European owners of Pokémon Omega Ruby, Pokémon Alpha Sapphire, Pokémon X and Pokémon Y can get Zygarde for free via Nintendo Network from now until 26 May. This powerful Lv. 100 Zygarde will be distributed with the Aura Break Ability, and it knows the moves Land’s Wrath, Extreme Speed, Glare and Outrage.