Reading Britannia's ground service manual it states that only staff with a Britannia issued ID will be permitted to travel on the flight deck. They can only travel on the flight deck whilst on duty and are not permitted on the flightdeck for flights to the US ad Canada.
It is a shame because I managed to get on the jumpseat of a Britannia 752 for a flight form CWL to AGP in 2002. I had a wonderfull time and the crew were brilliant.