To give you a little more - if the routing that day is initially over France the departure will be to the south coast (Bognor/Eastbourne way). If, as is more usual, it is via Belgium then via Dover.
If you really want to know, spend a little time that day using one of the tracker sites and see where flights to Greece/Turkey etc are routing. It still will not cope with an 'oddball' route, but you will get a general idea. You could always try asking the c/crew (if they have time) to ask the pilots before departure although I cannot guarantee the reception you will get! The PA might even tell you.