In the UK Hertz used to have a little Toyota Avensis they rented with a built in Toyota Navigation System. It worked great. Now Hertz doesn't have the Toyota anymore and you will have to rent a Benz or Jag to get a navigation system through them.
Avis cooperates with a company called Cellhire. You have to reserve separately, but the portable NAVMAN unit will be waiting with Avis when you collect your car. It cost 8 Pounds/day and can be a little frustrating at times.