Never having ever used a Sat Nav before a friend reccommended that I purchase a Mitac 510E (which has what you want). I purchased the unit from www.dabs.com fot �150 and am very happy with it. I do find my driving has improved, if only by the constant reminder "you are over the speed limit". It also has bicycle mode and pedestrian mode.
So two people .... two opinions ... as the day gets longer so will the list.
I also have a Tom which is excellent
but some have inbuilt mp3 players
you can get PDA versions
and the latest smartphones are good too
nip down to halfords or PC world ... or any of the big electricals and have a play.
or browse http://www.pocketgpsworld.com/
and see what the freebies are like ... speed camera locations are a must!!!!!!!!!!!!!