In early 1941, the British and Greek commanders did not trust each other. It also seems that the British and Greek commanders had problems communicating in that communications were often misunderstood.
In early 1941 the British wanted to be able to cooperate with Yugoslavia and the Greeks agreed with this goal.
General Papagos the Greek leader in early 1941 decided to hold Salonika at least until they had learned what the Yugoslav intentions were Salonika was the port that could be used to supply Yugoslavia. After the British commanders saw how the Greek army was deployed, they decided to hold a line along Olympus and the Vermion, as it seemed to be very defensible.
Once the coup happened in Yugoslavia the British considered trying to defend Salonika. They gave up that idea after being unable to cooperate with the Yugoslavian army.
Before the Greek campaign even started, the British commanders had no confidence in the Greek army. That was partly influenced by General Wilson's lack of understanding of the situation. He thought that the Greeks on the left had "disintegrated" when the real problem was to the right of the ANZAC Corps.
This is based on the account in "Greece Crete, and Syria" by Gavin Long.