I used to use a Garmin Forerunner (until I lost it) – good for measuring distance and seeing where I’d been on the map. Once saved my bacon when I could back track a biscuit crumb trail even though there was no map.
Now I use a Satmap Active 10 – has full OS mapping (expensive tho). You can download routes, but it doesn’t do turn by turn as it stores the map as a picture.
When I forget it, I use mapmyrun on my Iphone to record distance etc
Then put it all into Sporttrack.
GrahamA is right, a posh bike computer that doesn’t need wires