bealine: Miami is also an old Gatwick route, so it doesn't much matter where they stick it!
Only half true? MIA has long been an LHR route. What happened was that it used to be one a day ex-LHR and one a day ex-LGW, but the LGW frequency was moved to LHR. The current timings mean that the two frequencies comfortably share one gate at both ends of the operation.
IB4138: the additional security screening area before you enter FC from T2 and T3(which no one can explain the need for, as pax are already airside)
But the vast majority of pax entering the FCC have not been screened at LHR, having just got off inbound flights. If you didn't screen them again at the FCC, they'd just carry on the live ammunition that IAD let them through with.