Even a powered aircraft only needs an altimeter and a turn and slip indicator.
Not so.
There are two layers - the ANO specifies the equipment required in UK airspace, but the aircraft must also be operated in accord with the POH and the aircraft manufacturers limitations.
There is nothing in the ANO to prevent you doing 6G aeros in a Warrior but you had better watch out if you do - fortunately the POH specifically prohibits such activity.
Most manufacturers will not certify an aircraft for IMC operations unless it fulfills their quipage specifications. For example consider the DA40 range which comprises aircraft with three avionics fit, day VFR, day VFR night in VMC and IFR.
I dont know what the average glider POH says - is a glider specifically approved by its manufacturer for operations in IMC, or are the manufacturers silent on the matter, or do pilots ignore the POH and operate illegally?
There is one further layer - perhaps the most important - it is called common sense.
It is for that reason that I think you would be mad to fly cross country in IMC in a glider without some means of telling other airspace users you are there, in the same way you would be just as mad with an IR and an aircraft appropriately equipped to fly an ILS to minima cold turkey. Of course every so often someone tries - trouble is you only get one chance to get it wrong. They usually kill themselves.