Although the DfT produced guidelines, I believe however that ultimately the airline concerned has the say on if they are going to allow fam flights etc.
Apparently there are cargo operators in europe who are already allowing family members on jumpseats (a Spanish one and a Danish one I believe) .. do correct me if I am wrong however.
I really hope that the current ridiculous system of apparently not allowing family members to travel, whilst allowing other unknown people on the jump seat, doesn't continue forever. Somehow I can't see things changing soon!
Even if the restriction on family members is not going to be changed, I really think the restriction on ID carrying, ticketed ATCO/ATSA fam trips should be lifted and if so it would be of huge benefit to all parties.