By Andrea Shalal CHRISTIANSTED, St. Croix (Reuters) -G7 leaders will discuss a new legal theory that would enable the seizure of $300 billion in frozen Russian assets when they meet in February, two sources familiar with the plans and a British official said on Thursday. U.S. and UK officials have worked in recent months to jumpstart efforts to confiscate Russian assets immobilized in Belgium and other European cities, and hope Group of Seven leaders agree to issue a stronger statement when they meet in late February, around the second anniversary of Moscow’s invasion of Ukraine, the three sou…