LONDON — The wife of a British aid worker held hostage by the Islamic State group has issued a statement pleading for the militants to release him and respond to her messages “before it is too late.” The Islamic State group, which has released online videos showing the beheading of two American journalists and another British aid worker, has threatened to kill former taxi driver Alan Henning next. Henning, 47, was kidnapped in December in Syria, shortly after crossing into the country from Turkey in an aid convoy. His wife, Barbara, implored the militants to “see it in their hearts” to release him in a statement released by Britain’s Foreign Office late Saturday. “Alan is a peaceful, selfless man who left his family and his job as a taxi driver in the U.K.