I thought gliders were only allowed to fly in IMC either OCAS or in RA(T)s (normally poking up through Class A into the Class C).
In the first case, the UK IFRs provide virtually no collision avoidance value (and in fact forcing all aircraft of various speeds to into narrow altitude bands rather than random altitudes statistically increases collision risk). In the second case, the airspace is given over to glider ops and enroute powered IFR doesn't use the airspace.
As such, a glider in Class G IMC is no better or no worse than a spam can not squawking (and only marginally worse than a mode A only - and even then, only if the aircraft about to hit it has TCAS or PCAS or is in receipt of a radar service).
As far as I am aware gliders in the normal course of events are not allowed to fly under IFR in CAS nor are they allowed to shoot approaches under IFR (and Newton seems to have a law about them executing missed approaches
). As such, it seems perfectly reasonable that they don't meet the certification requirements for IFR operations (which for powered is either certified VFR only or certified for ALL IFR ops (RNAV, MNPS, RVSM excluded)).
I am not a glider pilot, but my understanding is they sink at a much slower rate in a spin, are less likely to overspeed in a loss of control, are easier to maintain up right in IMC, and in the end, are reasonably easy to bail out of and likely to do minimal damage on the ground - so all an all it seems reasonable to fly in IMC with less training than a full IR or IMCr and a parachute.
I do have sympathy for the PFA types which are fully tricked out with highly capable systems but due to their nature are not allowed to operate IFR in the UK and would agree, there doesn't seem to be a problem in States that do allow these aricraft to operate IFR.