Just used LGW for a trip to Egypt. Didn't want to risk the M25 on morning of flight, so got a hotel and parking package (using holidayextras website) in the Travelinn. It's a fair way away (4/5 miles) but the deal is you park at the hotel overnight, take car to car park in morning, catch shuttle bus to either terminal and return from trip straight to car park. All worked very well. Car park is close to North terminal, don't know how long to South.
UFO
BTW, got there plenty early, registered for IRIS and no queue for immigration on return - brilliant. Still have to wait for luggage, though.