A lead line coupling is a compression fitting designed to connect old lead water pipes to new copper or other modern pipes, commonly used for water supply or central heating systems. These fittings are heat-free, solderless, and allow for fast and simple installation, often with WRAS approval for use in hot and cold water systems. 
How it works
Joining pipe types: A lead line coupling has one end for the lead pipe and another end for the copper pipe.
Compression fitting: It uses a compression mechanism, rather than soldering, to create a secure joint. This involves tightening a nut that compresses a ferrule (a ring) onto the pipe.
Solderless installation: This means no soldering is required, making the installation faster and easier.
Dismountable and reusable: The compression fittings can often be dismantled and re-assembled.
Works on wet pipes: The fitting can be installed even if the pipework is still wet. 
Key features
WRAS approved: Many lead line couplings are WRAS approved, which means they have been certified for use in contact with drinking water in the UK.
Heat-free: The installation process does not require heat, which is safer than traditional soldering.
Fast installation: They are designed for quick and easy installation.
Dismountable: Joints can be taken apart and put back together.
Versatile: They can be used for both hot and cold water systems and sometimes for central heating.
Joint compounds are not needed: Avoid using jointing compounds or sealants as they can damage the fitting.