I've done a bit before when I was doing a lot of hiking, but I've not done any with a bike yet. (That will probably be changing next month
)
If you want to avoid the legal worries, head down to Dartmoor as it's legal there*. Otherwise I've only ever been hassled while in the Peak District where the wardens are very hot on discouraging wild camping (we were scouts practising putting tents up and obviously had no sleeping equipment but the warden refused to accept that we wern't planning on camping).
Look for a sheltered spot, preferably above the wall/hedge lines and try and be discreet (arrive late, leave early). Generally try to leave the place as you found it.
The main differences between wild camping and camp-site camping is the absence of water and toilets, so be prepared for this (trowels at the ready
)
If you get chance to wild camp, do try it. The isolation compared to camp sites means you really feel like you are cast away in the middle of nowhere, even if that's only half a mile from a tarmac road
* Part of Act that made the National Park
{eta}
It isn't illegal everywhere in the Uk...
No. It's legal in Scotland and in Dartmoor. It is legal in the rest of the UK if you have the landowner's permission, and in most mountainous areas it is tolerated if you are considerate