Any idea what could make a deep roar in St James’s Park at night?

Sound can be hard to locate in a quiet park, especially at night. A noise from a bird, animal, person, or nearby street can echo or carry in a way that makes it seem much deeper and closer than it is. Without a recording or a clear direction, there’s not enough to pin it on one source.

People can make surprisingly convincing animal-like sounds, and a shout or performance from somewhere nearby might come across as a roar when it echoes. I wouldn’t jump to anything supernatural from one unexplained sound. It may simply have been impossible to identify from where you were standing.

Could be an animal, but I’d be cautious about naming one. Nighttime calls can sound very different from what people expect, and distance, buildings, trees, and other surroundings can distort them. Being familiar with countryside wildlife doesn’t necessarily make a sound in a city park easy to identify.

That sounds frightening, but the reaction makes sense when you hear something loud and unfamiliar in a dark, quiet place. If it happens again, get somewhere well lit and populated first. A short recording from a safe distance, plus the time and exact area, could help someone identify it later.