I can only speak for BA, the revised policy for use of the flight deck jumpseat is very restrictive (in accordance with CAA rules). However, I quote "It is BA policy to support the international practice of carrying representatives from Air Traffic Services or similar bodies on route familiarisation flights where it is BA's view that the individual will benefit from observing take-off and landing procedures from the jump-seat".
Contact BA Flight Operations department to get the low-down on the procedures you must follow to arrange such a flight.
In my experience, the rules are VERY well adhered to in order to prevent any un-authorised access to the flight-deck.
Hope this helps.
F33