The post event minicab pick up area and the roads in the immediate vicinity have looked somewhat chaotic when I've seen it (I've not done it though), so I'm not sure a pre-booked car is necessarily a better idea - locating it and then getting out of the jam along with everyone else dancing on the ceiling might take a little while. I suspect the pros who have co-operative minicab drivers probably go elsewhere such as outside the Pilot pub a bit further south on the peninsula or at the bus stops on Millennium Way (as opposed to actually in the bus station) next to North Greenwich station - I've certainly seen people being picked up at the latter (it's double yellows though so they're not really supposed to wait there...)

The other thing is the road route to Marylebone - if the Limehouse Link tunnel is closed for maintenance then the alternative roads into town on the north side of the river can get very busy. Meanwhile the route south of the river into central London isn't particularly nippy. I'd go with the Tube.