score:1
How early do I need to be at BHX for the "non-transatlantic" leg?
I would do 45 minutes. 30 might be enough but that's cutting it extremely close.
I'm asking as I'm not sure when the screening/pre-immigration for the US will take place, at AMS or at BHX. I cannot see how it could possibly happen at BHX, but you never know.
You are right: it doesn't. If you have a documentation problem, KLM will refuse to check you in and tell you to see an agent at the airport for documentation check so you will know (you have no idea how many of these error messages I got for eight long years...). The US extra fun always happens as you enter the gate just before you board to a plane to the USA.