The British forces in Greece had little confidence in the capability of the Greek army. The reality was somewhat different. General Wilson and his staff tended to Blame the Greek army when the failure was due ro General Wilson and his staff. General Wilson ordered a withdrawal from the Aliakmon Line while saying that the Greek army had "disintegrated" when the truth was that the real problem was to thr right of the ANZAC Corps. The corps seems to have been too weak to hold their position in the face of the forcs that the Germans had ready.
By the time that the ANZAC Corps occupied the Thermopylae line, the Greek army in Epirus had "surrendered to the Germans. That German force made no move against the Thermopylae position. Again, the truth was that the weakened ANZAC Corps was not strong enough to hold against General List's army.
As for the withdrawal, General Wilson and his staff had waited too long to bein planning. It was also true that General Wilson had sent to commanders and their staffs to leave Greece when they should have been used to plan the withdrawal. General Wilson and his staff lacked the judgement and experience needed to command the Greek campaign. Wilson's main qualification to command in Greece was that Churchill knew him.
This is based on the account in "Greece, Crete, and Syria" by Gavin Long.