Well, Horizon started out as a partnership between E.On and RWE and was then sold to Hitachi in 2012. The (inevitable) choice of the ABWR was only made after Hitachi bought Horizon, so it was well behind EPR into the GDA.
NuGen was set up as a joint venture between ENGIE, Iberdrola and SSE. SSE bailed out in 2011 and later Toshiba bought Iberdrola’s stake plus some of Engie’s to give t a majority hoding. At that point they (inevitably) chose the AP1000 design and started that in GDA.
So EdF are first because they are the only one of the three which hasn’t had major comercial upheaval on the way to choosing a design, apart from Centrica, a minor partner, getting out early on. It was also clear from the beginning that Hinkley would be EPR so there was no technology choice to be made.