Straight from Pokemon.com:
Visit participating GameStop stores from March 4th, 2013 to get the Mythical Pokémon Meloetta!
Keep checking Pokemon.com to find out how you can add Meloetta to your game via a special distribution event at GameStop stores. You'll need a system in the Nintendo DS or Nintendo 3DS family, plus a Pokémon Black Version, Pokémon White Version, Pokémon Black Version 2, or Pokémon White Version 2 game. This Meloetta character distribution will also be available in Canada at participating EB Games locations starting March 9th.
The distribution event will run for a limited time—so don't miss this opportunity to get Meloetta!
Just to post official statement to avoid further confusion.
No they haven't posted anything for the UK or any Europe Countries. I'll keep checking their versions of Pokemon.com though Serebii will most likely post before I see the updates.
EDIT: Australia and New Zealand have their dates revealed. Both are on March 4th to April 19th. So they get it longer. In Australia it will run in EB Games and Target while in New Zealand it runs in select JB HiFi stores