It's built on a stepped structure made from chalk, it's not just a pile of earth.That’s still just a mound, not a pyramid. It has a perfectly circular base.
"Silbury Hill is a prehistoric artificial chalk mound near Avebury in the English county of Wiltshire. It is part of the Stonehenge, Avebury and Associated Sites UNESCO World Heritage Site. At 39.3 metres (129 ft) high, it is the tallest prehistoric man-made mound in Europe and one of the largest in the world; similar in size to some of the smaller Egyptian pyramids of the Giza Necropolis."
Silbury Hill - Wikipedia