How does the UK MEF event work once it is played, and what does the MEF need before it can move or attack inland?

Question

Accepted Answer

When the event is played, the MEF army and the Beachhead marker go on one of the four MEF spaces, which then counts as an Allied port. Activating the MEF to move or attack inland costs 3 OPS.

Paths of Glory

How does the UK MEF event work once it is played, and what does the MEF need before it can move or attack inland?