What are these connectors on UK plug socket internal wiring?
These are a type of splice that in your case was a safe way to terminate the wire in the bottom photo. In my area they are referred by there brand name Wago. I only use these types of splices or connectors in fluorescent fixtures but they are or can be used as they are a listed connector. Some contractors use them because they are quick to install and take up very little room in the box. I have had to replace many outlets and switches that have this same kind of connection called back stabs so I usually cut them off. If your wire is on the short side they can be crushed to release &amp; save the wire.