I think there is some confusion netween VMC and IMC comparing them to VFR and IFR. There is nothing to stop a glider pilot flying IMC in class G airspace in the UK. What he/she can't do is fly IFR.
I took my Bronze exam in 1980 and I remember the question being asked and I was very clear about the IMC rules. I have also soared into and climbed within cumulus clouds more than once and quite legally. In Australia where cloudbases are frequently above 10,000', cloud flying is rarely necessary. But it's a diffrent matter trying to fly a 300km task in the UK with a 3000' base.
For those outside the UK, you may be suprised to find there is no glider pilot's licence and gliders are not registered. The sport is self regulated by the British Gliding Association - a system that works. Unfortunately, this may change due to Euro harminisation and the sport will not be better for it.