I agree. Ordnance Survey make all the maps for the UK. You need 25,000 scale for walking, 50,000 for cycling or if you are just cycling on the road then you can get by with a 250,000 road atlas. I know on the iPhone you can get Memory Maps. It comes with 250,000 scale for free but you have to pay for the different scales of different regions. You can import a route you have made or someone else has made available online (usually a GPX file). There are others like Route Buddy which is iPhone and Mac only. These are not really sat navs though. I would still take a paper map and compass. If you do get lost then a GPS can be very useful as it can show you exactly where you are on the map.
You could probably get away with just a phone but I wouldn't trust it what with battery life and not being able to get a GPS fix when weather gets bad etc.