I have 25k mapping for the whole of the UK. It cost me about £130 a couple of years ago and has been more than worth it. But please don't discount the other mapping scales. I have walked in all areas of the UK using the 50k maps and before that the 1" map which was 1:63000 and both provide enough detail to follow a route. If the route is pre-planned and a gpx file put on the satnav then you only have to follow the yellow brick road with the satellite marker.
I would suggest that the 25k map is not enough for riding in urban areas such as housing estates where we now have cycle paths wandering through. I use Google satellite view to find these. I have also used the 1:10,000 maps.
