It depends on how many maps you want, if you want campsites marked or are prepared to research that in advance and how good your map reading skills are.
We have toured the UK on a standard £4.99 car atlas (1 inch to 4 miles type thing) before now without any problems, but we don't often venture into cities where I could imagine it would be less than helpful.
We have toured Scotland on 1:250,000 and also not had any problems and this seems to be the ideal map for us in places with less roads.
The standard OS Pink series (1:50,0000) we find are great for telling you what is around, but even I can cover 1 of those a day (though it is more often 2 every 3 days for a direct route tour rather than a round in a circle tour - but that still makes for a lot of maps and a lot of £££s) and I am neither a fast cyclist or go further than 50 miles a day when on tour!