It is illegal to undertake in the scenario you describe.
These are the only circumstances it is allowed (to undertake on left):
• When the driver ahead has moved out and given a right turn signal and you intend to go straight ahead or turn left.
• When you intend to turn left and have signalled this intention.
• Where traffic is moving slowly and the vehicles in the lane on your right are moving more slowly than the traffic in your lane.
Outside of these you may do like the rest of us, and sit & wait!