Transitting back from International flights via Heathrow to NCL you have to go through security again (albeit, after clearing immigration) although you remain airside, having been fully security processed at the departure airport (in my case recently on seperate occasions, Nice, Paris and Miami).
Duty free items bought abroad (liquids etc... which are over the 100ml plastic resealable bag rule) must be sealed by the Duty Free Shop at foreign departure airport, and remain sealed through the security / x-ray process at Heathrow)
NCL to Rest of World via LHR, From your Arrival gate. proceed through Flight Connections, having only your Boarding Card checked for validity, and then enter directly into the Departure Lounge/Gate area without going through Security scanning etc...