Rim Door Locks
A 'rim lock' commonly referred to as a 'yale lock' is a locking device that attaches to the surface of a door. Most Rimlocks used today on external door are nightlatches. Please be aware not all Rim Nightlatches are British standard so aren't approved by insurance.
Also bear in mind you can double-up your security by considering adding a deadlock further down the door at about waist height!