A glider pilot runs out of options when the lift runs out and a landing becomes inevitable.
This is something of an over-simplification of the glider pilot's decision process. Often the lift is getting weaker and the glider is sinking slowly. The pilot often feels that if he can only hang on a bit longer, the thermal will improve and he will climb away. Alternatively, by moving a bit to another position there may be better lift. At some point, the pilot must decide to abandon the flight and land. With a good field picked, that can be lower than when no field suitable for landing in can be spotted. Early cross country pilots are typically advised to commit to landing when they are down to around 1000' having started picking fields at perhaps 1500'. More experienced pilots are likely to continue trying to get away from lower heights. Some are even rumoured to have soared away from less than 300' AGL.
The truth is that most serious glider field landing accidents result from picking a field too late and / or trying to continue to soar away from too low a height with no time to execute any sort of sensible circuit into the field.
There's a lot in common between the decision to abandon soaring flight and land because of deteriorating lift and the decision to abandon powered flight and land because of deteriorating weather. The difference is that the glider pilot trains for it, and expects it to happen.
If it is true that in the case of this accident, the pilot had got his undercarriage down then one would assume the decision to land had been made, presumably at Popham. However, executing a decent circuit at low altitude perhaps flying into and out of cloud would never be easy.