So ITV hold the copyright, rather than Rediffusion and Thames's corporate successors?
It depends entirely who owns the rights to the programmes that the telesnaps are of. The bulk of programmes made by Rediffusion are owned by Archbuild. Others are owned by a variety of broadcasters including ITV. Thames material is now owned by Fremantle, the company that acquired Thames. Thames is now a revived subsidiary of the production wing of Fremantle.