USA early entry due to invasion of UK

[kennonlightfoot]

My understanding is that if the Axis invades England it will trigger an early US entry into the war. I believe it was something like Feb 1941. What I am not sure of is whether any landings in UK will trigger this or whether only taking a VP hex or London triggers it.

Can anyone tell me the exact trigger requirement?
And, does it also increase US production multiplier?