What is the most dangerous animal in the UK?

"Deadliness" is a subjective concept.

The animals we consider deadly to us will be harmless to many other species, and those that are harmless to us may be voracious predators in their own world.

Based on this premise, the UK's deadliest animal is quite possibly the Common Water Shrew which kills many times it's own weight of other small animals every day.

The deadliest animal to humans in the UK is other humans