I know there used to be quite a strict time limit for a/c departing north/eastwards with Terminal Control (TC) at Swanwick, I'm not sure if this is still the case, any of you TC south bankers in the know?
Also, I imagine the co-ordination sequence will go via Solent Approach so there could be a stipulation that the aircraft is on runway and ready to depart (not by TC but from Solent).
Don't forget that a SLOT may have been issued too just to narrow those goalposts slightly more.....