There is a void in T5C's roof area which can house a lounge (like T5B) but it hasn't been built yet. It will probably open once BA have a few A380s (as they will mainly depart from T5C and BAA's incompetence in spec'ing the transit train means that you'll need to hold some pax in C before boarding to avoid them all hanging around in A and crushing the train when boarding is announced).