It's not just a case of whether there an airport is full or not... When you have advance notice and time to prepare then There is no point sending the aircraft somewhere if there is not enough handling staff to deal with the diversions or enough coaches to transport the passengers (if the crew are then out of hours) or in some cases hotels. Another factor can be engineering cover, if the aircraft is flying with something inop under the MEL. Also another factor to throw in the pot is availability for replacement crew to then reposition the aircraft for its next revenue flight. At easyJet and most airlines we have designated diversion airports that are flight plan preferred and also company preferred (for the above reasons) some situations such as missing a night curfew means that the aircraft would pretty much always go to the number one preferred alternate as there are contracts and procedures in place to deal with the diversion. With events such as mass diversions ( Gatwick is a pretty big airport) it sometimes just comes down to where ATC tell you to go or as people have said where there is room to get the plane safely down on the ground.