Flashing headlights is used to alert other drivers of your presence.
If another car is flashing their headlights when you are waiting to pull out. This does not mean it is safe to pull out. It is your responsibility to make sure it is safe.
Never wave a car out. Do not direct traffic unless you are qualified to do so.
When police are directing traffic, you should understand the hand signals and obey them. You should know how to signal your intentions. Learn the hand signals shown in “The Official Highway Code”.