Great tits chicks

Hi.  My cat has killed both parent birds who were nesting.  Chicks were screaming for food.  I have them in a box and have been feeding them raw minced beef and they are surviving.  Is there a rescue centre in or near birmingham.  I don't know what to do when they are ready to fledge.  They are 2-3 weeks old I think.  Can anyone advise?  Thanks. Emma