Unsure if you meant a SatNav permanently installed in your car or not but if you are prepared to have a PDA type, go for Evesham's BM 6300 (link below). It's only £69.99 and has navigation and speed camera alerts. You get free camera alerts for six months, after that it's £30 per annum. I have had one of these for three years now and it's absolutely first class.
http://www.evesham.com/products/info...5-8CC410F533CA