Originally Posted by
MANFOD
had to join a long queue for security despite the check in the US.
This is a UK Foreign Office (not aviation) decision. There are other countries where they accept inbound arrivals from certain other countries, like the USA, to be adequately secure. The Foreign Office however feel that to give this credence to some countries but not others would offend the more touchy diplomats from certain places asking for this dispensation and being told "No", so the easiest solution for them was to have nowhere regarded as acceptable, and so to re-screen everyone other than those from domestics.