You may find that the rules and regulations vary between airlines, but the CAA has not imposed a ban as such on all travel on the jump seat full stop. In BA, to be able to use the jump seat, you must be either a family member or very well known to one of the pilots, take that position at the start of the flight and remain there throughout, only leaving for pysiological reasons.
If as you say it is a friend who has offered you a trip, and you are well known to him, and he is confident that you are a sane and sensible person, then provided the company allows travel within such restrictions I see no reason whatsoever for your jumpseat ride to take place. As scroggs says, you need to be a companion of the crewmember, or as close to as possible.