Rhythm Thief said:
1:50 000 will be fine, then. I'd research campsites on the interweb before you go, since they're often not on maps of any scale.
+1 for OS Landranger 1:50,000 maps. Blimey this is becoming spooky agreeing with RT

. 2nd time in 7 days .......
1:25,000 Explorer maps don't cover a wide enough area for my liking when cycling. As soon as I've cycled onto one edge I've cycled right across it and off t'other side in no time what so ever. The 2 maps I have are for the Eastern and Western areas of the Brecon Beacons. One is 30 squares wide (30km/18.75miles) and 22 squares tall (22kms/13.75miles) the other 30 squares tall (30km/18.75miles) and 20 squares wide (20km/12.5 miles) which means they do not cover a wide enough area for me if cycle touring. Ideal for hiking in a small area or slow cyclists.
The 1:50,000 maps cover a much wider area 40km/25miles (1 square = 1km) whilst still having more than enough detail on them for cycle touring. I have noticed quite few campsites on them. One can of course mark on campsites and other features you want to visit or passby.
I find the OS Travelmaster series 1:250,000 maps helpful for overall route planning as 1cm = 2.5km. The map is 30 squares across, each square is 4cm meaning each one covers 10km, so the map covers 300km/187.5 miles across. It still has quite a lot of detail in minor unclassified roads which one can always defer to the 1:50,000 map should one wish to.
I spend hours looking at maps of all scales. I love the Times Atlas of the World and my large globe.