There are staff security points in all the terminals but these are still busy and sometimes shut as BAA are short of staff and staff get pulled to cover the passenger terminals. Certainly happens down Terminal 4 way.
Years ago in 2000 when I worked at Terminal 3 for a year there was a security post tucked away in the departures baggage processing area. Could be airside in less than 2 minutes.
Don't think you could do that now days though!!