LONDON, July 7 (Reuters) - Britain on Tuesday summoned Iran's Chargé d’Affaires, Tehran's most senior diplomat in London, after two Romanian nationals were given lengthy jail sentences over the stabbing of a journalist working for a Persian-language media group in London. The two men, Nandito Badea, 21, and George Stana, 25, -who British prosecutors said were acting as proxies for the Iranian government - were sentenced last week to eight years and 12 years in jail respectively for their role