There is an impending change to the requirements in NPA 2014-29, although it will not usually apply to differences training in SEP or MEP aeroplanes.
FCL.710 Class and type ratings - variants
(a)....
(b) When the ‘List of type of aircraft’ published by the Agency requires differences training or when mandated by the OSD, this differences training shall be conducted at an ATO.
(c) Notwithstanding the requirement in (b), differences training for SEP and MEP aeroplanes may be conducted by an appropriately qualified instructor unless mandated otherwise by the OSD.
A cynic might believe that this is being done simply to protect the revenue streams of certain national authorities.