What I remember from army service - plastic explosive needs other kind of detonator, not a fuse, but ok, i'm not an expert in explosives.
Then, lavatory seems to be much safer place to ignite the fuse - nobody will see you and nobody will interrupt you. Also, it is much easier to take some baggage to avoid problems with boarding, isn't it?
Those guys managed to get British passport and explosives for him, but failed to instruct him on simple issues I've mentioned above? Understand my point - kamikaze-terrorists may be stipid, but not the guys who organizing terrorism acts.