OutDoors GB GPS iPhone app review
£2 gets the 1:250k map of the Great Britain version,or £5.50 the GB National Parks 1:50k maps, also great value. Route planning and editing is simple, but not 100% intuitive; you can plan and import routes from online, as well as download extra maps for a fair price.
All are stored on the phone and render very quickly when scrolling/zooming. You can also record routes on the go, and toggle between map and stats – like speed, height, grid ref, etc – quickly. It does eat the battery, though.