I use Trailwise (a Google Map with UK greenlanes on), though it can be a tad buggy on the iPhone (routes are loaded via Javascript - the more there are, the slower it becomes). I do wish there was an app available to purchase though.
Memory maps for iphone! Ive got memory map and tomtom sat nav on my iphone which is amazing ie I search for byways on my laptop with mem map and mark out routes etc then download them onto mem map app on iphone. I get the map coordinates from mem map and load them into tomtom app. Tomtom takes me to coordinates and then I switch to mem map and press button the show my location on map and hey prestow im at the beginnig of planned route!