http://www.avis.co.uk
Avis worldwide operates in over 4,000 locations in 114 countries.
http://www.hertz.co.uk
Hertz operates from approximately 7,700 locations in 145 countries worldwide.  It is the largest general use car rental brand in the world.
http://www.europcar.co.uk
Founded over 50 years ago Europcar have grown to become the largest European car rental company, with a fleet of over 200,000 vehicles at 2,825 locations in 143 different countries.