Well, according to PC Gamer the release date will be Marth 12th, 2013.
Eurogamer has reported that Heart of the Swarm well be released on March 12th, 2013. The information comes from Eurogamers Spanish site, who spotted a listing on Battle.net. An official announcement is expected later today.
Its uncertain at this time if the March 12th date is for a worldwide release or merely North America. What is known is that Blizzard will be selling a Wings of Liberty and Heart of the Swarm double pack for £45. The new game alone is listed at 39.99 in Europe, meaning it will likely sell at £30-35 over here.
If you are interested in the beta Raptr will release 650 keys for enter the beta. You only need to be Elite in Wings of Liberty for get the key. I suppose it will be first come first served.
Edit: You can already pre buy the game from Blizzard Digital Store.
Edit2: Official announcement of Blizzard.