General Wilson decided on 13 April 1941 that the British would not rely upon the Greek army for anything. Wilson checked with General Blamey and then decided to "withdraw to the Thermopylae line". The German air power was now very much in evidence. The Germans bombed a town and that did away with the civilian government along with theh police and telegraph. The railroads were also showing the influence of German bombing.
The British air power in Greece was now showing the effects of the German attacks. The British air force in Greece was losing its effectiveness. They faced losses in the air and on air fields.
Reports now suggested that the Greeks on the left were losing effectiveness. The British now worried that the Germans might push south along the Pindus towards Grevena and Yannina.
The British heard that Greeks from Albania did not want to join the line being formed and instead were heading for Athens.The British were thinking of heading for Thermopylae where they could hold a position that didn't need Greek support. Thermopylae was about one hundred miles south.
The proposed new line would include the Thermopylae, Brallos, and and Delphi passes. The plan had problems. One being that the Germans would be able to position air power within range of Athens. This is based on the account in "Greece, Crete, and Syria" by Gavin Long.