With 67 million people in 81,000 square miles and a 150 year history of cycletouring, I think surely anywhere one would want to ride in the UK has been done before and it's been documented in some fashion. Off the top of my head, there's the Sustrans National Cycle Routes



and the National Byway



There are innumerable coast-to-coast routes



and water-to-water routes like the Devon Coast-to-Coast (which curiously actually does cross Devon) and the Lôn Las Cyrmu across Wales



and the Scottish C2C, a north-south route from Annan to Edinburgh.

For a walk on the wild side, there's the Hebridean Way.




There are numerous documented & mapped routes from Lands End to John o'Groats (LEJoG). The Cicerone is pretty good.

There are guidebooks, published travelogs and even YouTube videos of these rides. Cycle UK offers local cycling guides:

https://www.cyclinguk.org/routes/cou...cycling-guides

Didn't bring your own bike? You can get a bike out of a vending machine in the UK!