'Instrument Flight Rules' means Instrument Flight Rules prescribed by the Rules of
the Air;
'Instrument Meteorological Conditions' means weather precluding flight in
compliance with the Visual Flight Rules;
This suggests therefore that flight outside of compliance to VFR (ie. when within 1500m horizontally or 1000' vertically of cloud when above 3000' altitude) is in IMC conditions. Flight in IMC should be conducted according to IFR. I cannot see anything in the above that suggests an exemption for gliders.
I'm not suggesting that gliders are not permitted to fly in or near to clouds. I just can't see how flying round in circles when thermalling (when above 3000' and within 1000' of cloud) is flight in accordance with IFR unless there's an exemption.