By moving to the new line at Thermopylae, they would be ceding Greece to the north to the Germans. They would still hold a peninsula that was some 35 miles wide. This was between Lamia and Athens. They would lack the ability to cooperate with the main Greek army. The Germans would have the ability to base their fighter aircraft within range of Athens.
Of course, the British were well-equipped with vehicles, unlike the Greeks. The Greeks would be forced to march for weeks to move so far.
General Wilson ordered the British to move against the Greeks to keep them from getting in the way of British movement. He was ordering the Anzac Corps to keep the Greeks out of the way.
When the order was given, the Anzac Corps was not in combat with the Germans. German aircraft were now attacking Greek and British forces along the Grevena road.
The situation was strange, in that there were secret plans by the British to withdraw from Greece. Wilson's orders were given early on 15 April. The British could not afford to wait to begin withdrawing.
Demolitions by the British were planned to try and slow the German advance. The British had four forces ready to provide cover for a withdrawal. They were the 1st Armoured Brigade, Savige Force, the 6th New Zealand Brigade, and the 19th Australian Brigade. This is based on the account in "Greece, Crete, and Syria" by Gavin Long.