21 April and later in Greece

 Back on 20 April, the men in the field had not heard anything about a pendin withdrawal from Greece. General Wavell drove up to Blamey's headquarters at about 2am on 21 April. They learned that the Greeks had surrendered and that the British were going to be loaded onto ships to leave. Wavell eventually gave General Wilson a written order to withdraw using the prepared plan. He would cooperate with the navy. 

They might end up "embark some Greek troops". 

British troops would carry as much equipment that they could. British troops should not surrender but should try to travel into the Greek islands, where they might eventually be rescued. The plan might end up failing so that British troops might need to implement the option to travel into the Greek islands. One of the backup plans was to rescue British troops from the islands.  

They would embark men from the island beaches or ports. They would plan on evacuating not just troops, but also guns and transport from the islands. 

This is based on the account in "Greece, Crete, and Syria" by Gavin Long.