The TomTom Go series have been advertised by making a feature of their versatility as a handheld devices, as well as for use in the car. Perhaps that's what you're thinking of? There are various models, starting from �249:
http://www.tomtom.com/products/category.php?ID =0&Language=1
However, the mapping used by TomTom only shows streets, not footpaths.
If you're a serious walker, I'd recommend buying a cheaper make of satnav for the car (at, say, around �150), then spending the spare �100 on a Garmin handheld navigation system which, in conjunction with an OS map, will be far more useful in the countryside:
http://www.argos.co.uk/webapp/wcs/stores/servl et/Search?storeId=10001&catalogId=1500000701&l angId=-1&searchTerms=0199375&x=687&y=56
Chris