The British decided to make the withdrawal as quickly as they could, starting at 15 April 1941. The British units were well-equipped with vehicles, so they could move rather quickly. General Wilson told the ANZAC corps to keep the Greeks from traveling routes that would slow the British movement. They could see that the Germans were massing forces "on the lower ground forward of the three main passes". The Greeks were able to still hold two the three passes that were near the Grevena road.
The Central Macedonia Army was retreating along the Grevena road. The Central Macedonia Army and the Epirus Army were still being allowed to withdraw. The German air force was attacking both the Greek and British formations, though, on the Grevena road.
The British were now seriously considering a withdrawal to the coast so that their troops could be picked up by ships and carried to Crete or Egypt. It was unclear just how long the British and ANZAC Corps could have held on the "Aliakmon and Venetikos" lines.
As it was, the British withdrawal started just in time to have a chance of success. The order was given at 9:30am on 15 April 1941. The responsibility for the command was given to General Blamey. That shows just how much confidence that the British had in General Blamey, thanks to how well he had done up that point. This is based on the account in "Greece, Crete, and Syria" by Gavin Long.