Stangmoor Prison was prison in England.
Before becoming a prison it was a fortress in the Middle Ages. It was the ideal place to demonstrate The Keller Process, as there were many criminals available. Later, when the Master and his band of convicts took over, it was used as base of operations for the plan to hijack the Thunderbolt missile. UNIT administered it for a time after it was retaken from the convicts. (DW: The Mind of Evil)