That's because London is on clay. The Reading guy was mostly on chalk (and was himself an engineer). It sounds like the water table was a long way down as well. Soft clay or waterlogged sand are more like digging a hole in a bowl of custard. There are a lot of things that can go wrong - in the case of clay, even if you don't collapse the hole the clay can expand for years afterwards ("heave").

Doing underground expansion correctly won’t cause streets to sink.