I guess the trial is as much to get ATC used to it as anything else.
There is no objective need to trial GPS; the Americans have done that over 1 million + GPS approaches over a decade or more.
I wanted to test the GPS sensitivity sequencing because while anybody can fly a GPS track by following the track line on the moving map, it is useful to be able to fly it coupled to an autopilot. Just like it is useful to be able to fly a coupled ILS occassionally, on days when one is under a heavy workload.
The sensitivity increase can potentially play games with the autopilot's ability to track the track, especially at the 0.3nm full-scale setting, inside the FAF.
I don't think the CAA thought about this aspect of the matter. By limiting it to G-reg, they have limited it (mostly) to pilots that don't have the equipment, or they have it incorrectly configured, or (and this will be true in nearly all cases) they don't know how it's supposed to be used to fly a real GPS approach.
To top it, CAA inspectors placing a plane onto G-reg for the first time have generally insisted that an IFR GPS has approaches disabled (in the maintenance config pages, not officially accessible to the pilot), which will prevent most new G-reg planes from participating in the trial at all. I don't think the CAA thought about that too; that their own CofA inspectors have unknowingly sabotaged the trial.