Churchill wrote about how he thought that the British evacuation from Greece should be handled. He wrote that note on 18 April. Churchill's participation in Greece had been colored by his interaction with Anthony Eden, who had been a strong proponent of going into Greece. Anthony Eden thought that the British had a strong moral obligation to help the Greeks. Anthony Eden thought that they had might have a legal oligation, based on treaties.
Churchill was encouraged by how well the withdrawal to Thermopylae was proceeding. That had colored Churchill's letter to Anthony Eden on 20 April.
The Australian historian commented that by 20 April, it was too late to think of making a stand at Thermopylae. While all this was considered, there was a change in the Greek command, where General Pitsikas was removed.
By early on 21 April, General Wavell arrived at Blamey's headquarters and told him that he needed to evacuate his units "as soon as possible".
Blamey would need to get his troops to the beaches by 24 April. They would need to board ships by 25 April. This is based on the account in "Greece, Crete, and Syria" by Gavin Long.