Mortice locks, commonly known as "Chubb Locks" are usually found on wooden doors supporting a night latch, and are some of the most secure locks available. Modern British standard versions come with anti drill plates protecting the internal components and lock a 20mm strengthened bolt into the door frame.
Did you know that 40% of people with mortice locks leave the key in the lock behind the door. This is a burglars dream! Keys left inside the door can easily be turned from the outside with some relatively simply tools. British standard locks use a curtain to interact with the bolt while the bitting on the key lifts the levers to the correct heights. This curtain holds the key in place when in the lock, meaning all the thieves have to do is turn the curtain and the key will follow.
Don't make it easy for the thieves to get into your home, keep the key out of reach of the door.