With all due respect Shed there is a big difference between US forces on a base or on exercise, to armed Customs officials on duty at an airport. There is enough controversy about arming our own police forces.
The US may have spoken to DfT and received a warm response: let's just see how the Home Office responds.
I do wonder if this is a driver behind the expected terminal redesign. Having to develop three separate sterile areas (I.e. one per terminal) can't be an attractive proposition.