If you want the professional gear, I hope that you've got deep pockets. (You probably won't see much change out of �1000).
Individual ambulance trusts are free to choose their own preferred make of satnav. However, the vast majority use VDO Dayton systems: http://satnav.org/
Scottish Amublance Service use sat nav and mapping systems from one of my customers - Terrafix Ltd. Professional kit like this is beyond the budget of the public I'm afraid.
Anyway, it should be noted that these integrated systems have much more than sat-nav. They are full mobile data & comms systems as well as tracking and location.