Regarding LGW I am really surprised your asking questions rather then giving full answers as your accustomed too.
It's how you learn... Last time I got off Pier 1 at Gate 5 there was a piece of equipment at the top of the bridge labelled BAC111 use
EZY have a subset of arrivals at each terminal which arrive at the opposite one so I assume not all are bussed and some are allowed to deplane at the "wrong" terminal. There's a FIDS screen especially in arrivals for meeters and greeters to let them know.
The original (rough) plan was long haul at North and (mainly) loco and charters at the South, with Virgin staying put due to the Clubhouse. The rather spectacular rise and rise of easyJet has meant that they are the exception. I suspect they'll remain split for the foreseeable future, the game changer might be BA getting a third runway at LHR but that's not still a mid term option.