Neuengamme, May 4, 1945
The Germans had evacuated the camp just days before the British had arrived.
Bergen-Belsen
Bergen-Belsen was liberated by British forces on April 15th, 1945. Around 60,000 were discovered in the camp, most of whom were seriously ill. After the camp's liberation, another 13,000 former prisoners died because they were too ill to recover. When evacuation was complete, the British burned down the entire camp to prevent they spread of typhus.
Main source:
http://www.ushmm.org/