Another reason why the Gatwick operation runs so smoothly (apart from the great controllers ofcourse
) is because they are afforded the luxury of having a relatively large chunk of airspace to play with. Being the southern-most airport in the london TMA enables them to be really flexible with the inbound stream down to the south of the field.
It also helps that KK's runway is kindly alligned with EGLL's for more-straightforward airspace and procedure design. Stansted, another London single runway airport, have a bit more of a hard time mainly because their runway is a sw/ne orientation causing the design of airspace and procedures to be very awkward alongside the other London airports.
I also can confirm that Gatwick's (and every other London airport's) holds are each filled with aircraft depending on the inbound route rather than the weight.