I've lost count of the number of times a pilot (usually not an airline pilot) has commenced departure after receiving a departure clearance (route and level to follow) but without a takeoff clearance.
Are you aware of this incident?
There was a case of a Lufthansa crew at London who commenced take off after receiving a delayed airways clearance. So confident were they that they had received a TO clearance, they radioed back ATC telling them it "wasn't a good idea to have given a TO clearance with other traffic so close." !!!!
cheers