Get an OS map, find the small streams high up in the hills and look for a loop in the contours where they open out. This will mean a flat spot with water, away from farmers 🙂
The exception is forests – they contain lots of flatter spots which are good for biviying if not camping, and they provide lots of cover to hide from landowners. And you often end up face to face with wildlife 🙂
EG see the dead centre of the map at this link:
http://wtp2.appspot.com/wheresthepath.htm?lat=52.41097339016394&lon=-3.768010139465332&gz=17&oz=8>=1
Might not be any flat ground there but it looks promising, and you don’t need much for a bivi. For this reason of course it’s good to have options in case your planned spot turns out not to be suitable for some reason or other. And an ortlieb bag in your pack so you can go get water on your bike if you can’t find a nice spot near a stream 🙂