Ah .... they're not mice. All of Whitehall is build on a swamp, with Westminster Abbey and Palace on an island of rock. Lots of drains, tunnels, abandoned and half filled in foundations .... you get rats. London sewer rats, a special breed. About as long as your forearm, not counting the tail. They are the very good reason no one sleeps rough around Whitehall. Not the cops giving hassle, it's the waking up missing toes, fingers, and noses that is off putting.