I don't know much about Sat Navs but I thought this could be a pretty good deal if you are looking for something simple.
Binatone B430 UK Sat Nav Refurb
With slim-profile and wide-screen the B430 is an excellent choice. It comes with all basic GPS functions satellite navigation, voice-guidance, postal code search and speed camera alert. The device is small and light enough to fit into a shirt pocket or bag when not in use, yet inputting a destination, and viewing the maps, is always clear and easy.
4.3 inch LCD
Full UK and Ireland mapping
Full UK 7 digit postcode search
Clear turn-by-turn voice guidance
User friendly menu