I would think the airframe would work rather well as a Faraday cage, effectively blocking most radio transmissions from getting through, so I would imagine you would struggle to actually get any RF through it. This is why antenna are mounted on the exterior.
Not sure on the transmission pattern of the antenna used for ATC but I would think they are capable of being recieved directly above the station, wouldn't make much sense if RT to the ATC cut out directly above the airfield!
As for taking a scanner on in hand baggage, I thought all electrical/electronic goods had to be placed in hold baggage? Or has that rule been relaxed?