Welcome to Flyertalk and the BA board
A380 trollies I believe are always manned by two crew (something to do with lighter trollies IIRC?)
Were you travelling during a strike period? MIA is a Mixed Fleet route and they have been operating certain flights with minimum crew.
There are no crew based outside London (well, certain international/overseas exceptions but let's stick to that for now). The crew would have likely operated a flight or two before LHR-MAN, overnight in MAN, then pick up the first flight back down the next day.
The middle seats free is thanks to Theoretical Seating - there are a few threads about that kicking around. Been about a year now.