Game Freak has announced a new way to download event Pokemon in Pokemon X and Y. As announced on the official Pokemon website in Japan, you'll be able to enter serial codes to download special Pokemon that aren't always available in the game.
So, as long as you have the Pokedex, you'll be able to enter the code and obtain your Pokemon via the Mystery Gift feature which is now available from the beginning.
Serial codes were available for the Pokemon Black and White games and could be used within the Global Link but not for downloading Event Pokemon.
As well as this, you'll also be able to download event Pokemon via Wi-Fi when you go to a Nintendo event or a store that is hosting a distribution (like the current Legendary Creation Trio event at GAME stores). Serebii reports that some Wonder Cards will also be distributed via Wi-Fi so you'll be able to get them without visiting a store or an event. The Pokemon site also reports that you'll be able to get hold of Pokemon via Infra-Red. Apparently these will be used at some events in Japan.
It remains to be seen how many distribution methods Nintendo will use to distribute event Pokemon in the UK during 3DS' lifetime. So far, we know that you'll be able to get Torchic on 12 October via Nintendo Network. You'll need it if you want to get hold of Mega Blaziken.