It has come to light through a few StreetPass groups around the UK that Nintendo is planning to a hold a Pokémon tournament for Pokémon X and Pokémon Y owners across the UK.
The finals for the event will be held in London on a currently unannounced date in a currently unannounced location. To qualify to take part you must first take part in one of the qualifier tournaments ran by StreetPass groups across the UK and take one of the top three spots in the event. Due to short notice and a bit of a lack of information, a couple of the qualifiers have already taken place, but fear not as there are still quite a few more being held across the month!
The rules for each qualifier do differ slightly. All events will be double battles with flat rules, but some limit you to only using Pokémon found in the Kalos Pokédex while others will allow you to use any Pokémon catchable in Pokémon X and Pokémon Y. The actual rules for the finals have yet to be confirmed.
Havak over at NuggetBridge has been running around finding out information about the qualifiers and has written up a nice list of them with all the information you could need right here. Not all qualifiers have been confirmed yet, so the list is being updated fairly often.