Your two hour limit rules out most of the SW coast path, Dartmoor and Exmoor!
But the Beacons and the Forest of Dean are within your reach.
You will have to stealth camp, but that's rarely tricky. Look for public footpaths that run through woodlands. That at least gives you the right to be there (if not to camp), and you rarely have to stray far from a path in a wood to become invisible. Generally folk don't go wandering in the woods after dark, so if you are not perfectly hidden you'll be fine if you pitch late and leave early.
To research routes I use OS maps to identify paths and woods, then go to Google Earth to have a good look. The size of trees can be gauged by the length of shadow they cast and the size of each trees canopy from above.
In addition to looking for circular routes, consider using public transport to top and tail a linear route.