It took delivery of five of the vehicles at the start of the year and has now ordered two more.
The group operates a user-chooser policy based on 12 manufacturers and encourages staff to think green when choosing a car.
The buying, IT and finance departments of the company’s new head office in Middlesex will use the new cars.
Fleet manager Linda O’Hara said: ‘Drivers, particularly the accountants, are attracted by vehicles that offer tax benefits.’
Staff will also now be able to escape the £5 London congestion charge by using a Prius.