When you buy an Ordnance Survey Map now it also comes with a free mobile download of the same map
It is easy to download from the OS site you just need to set up an account (free of charge) and then enter the code that comes with the map and then download it to your phone – either and iPhone or an Android.
And then you have a 1:25,000 map on your iPhone with full GPS functionality – i.e. your phone will show you exactly where you are. You can also plot routes to follow are record exactly where you have been.