It depends what style of riding you want to do really. There is an extensive network of National Cycle Routes, but the terrain varies. It could be quiet roads, old railway paths (either paved or gravel), canal towpath, fields, some single track, etc all along the route. The Sustrans website will give you the lowdown as to the type of terrain you can expect to encounter. Cycling UK has some good information and are releasing new routes every year. I tend to use Komoot for my planning. The cycling UK forum and cyclechat forum are mostly UK based/biased and will be helpful. Where are you looking to start/finish your ride?