Before we even pushed back at LHR, the Delta Police systematically went down the plane closing all of the blinds. A few protests ensured but Delta Police insisted it was a security requirement.
I thought it was a requirement to have the blinds open for take off. Maybe you did get a reprieve for the take off but I do get your frustration.
Guess which US airline I never fly with now
Delta although I think there are reasons to avoid all US carriers over the Pond.