As a member of HM forces and a previous truck driver who has driven all over te uk here are my thoughts, for what there worth.
This is a bit of an open ender as it depends on a number of factors. What area do you intend to cover, how much detail do you want etc etc. the best maps of the uk are the OS 1:50000 series. These allow you to navigate to within 25m and show a variety of surface and unsurfaced rights of way. With experience you can judge what the land will look like and whether it is scenic etc. however each map only covers, I think 100x100km so are quite limited and expensive for long range touring. Any standard road atlas will show B roads. Whilst 1 1/2 miles to inch show good detail they are too large for easy route planning. Conversely 4 miles to the inch are rather too small and 'busy'. Paersonally I am a big fan of 3 miles to the inch as a good balance. I am a devotee of B roads and find these ones fine.
1990 Landcruiser H60. Full rebuild completed 2014
2012 Hilux, mountain top and bedrug with GG AT2s